FTP client/server failing on switching to PASV mode

Update The ISP said that they recently added the area where this computer is located to their IPv6 network. Coincidentally, ever since the day they added that area to their IPv6 network, no FTP connection was possible.

The ISP was able to set this specific connection back to IPv4 (for a limited amount of time) and...FTP worked again.

Original descirption of the problem

I've a TCP level problem between FTP clients and FTP servers (I think it's a proftp server but I'm not sure).

Upon requesting PASV mode for data transfer, the client receives an ACK but does not receive the 227 entering passive mode message. The client waits for 30 seconds before it sends a TYPE A request which the server tries to ACK, but with the wrong SEQ number.

The SEQ number contains the size of the Enter passive mode message. The server also sends a 200 Switching to ASCII mode which also has the wrong SEQ number.

From there it's 7 tries of TCP Retransmission and TCP DUP ACK while both sides are using 'their' SEQ numbers. (Shouldn't the unconfirmed message that the server sent be retransmitted? Meaning, shouldn't the server just sent the 227 entering passive mode message again? Or at least fix the SEQ number?)

I got multiple questions here. For one, which side is behaving wrong here? One side has to fix it's SEQ number, but who? And how can a specific message (it's always the Entering passive mode message) always get lost?

To make matters more strange this is happening only on one computer. Out of hundreds. And it's also only happening with one FTP server. (which I don't have control over). The server and the computer in question are connected via the internet.

The logs were acquired via the client's router, so it's not a PC problem.

Ok, I managed to filter the protocol. So here it is:

No.     Time        Protocol Length Info
     39 5.988043    FTP      68     Request: PASV

Frame 39: 68 bytes on wire (544 bits), 68 bytes captured (544 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 68, Ack: 294, Len: 6
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     40 6.034184    TCP      62     ftp > 49736 [ACK] Seq=294 Ack=74 Win=32695 Len=0

Frame 40: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 294, Ack: 74, Len: 0

No.     Time        Protocol Length Info
     41 36.381924   FTP      70     Request: TYPE A

Frame 41: 70 bytes on wire (560 bits), 70 bytes captured (560 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 74, Ack: 294, Len: 8
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     42 36.417678   TCP      62     [TCP Previous segment lost] ftp > 49736 [ACK] Seq=345 Ack=82 Win=32687 Len=0

Frame 42: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 0

No.     Time        Protocol Length Info
     43 36.435735   FTP      92     Response: 200 Switching to ASCII mode.

Frame 43: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     44 36.436237   TCP      62     [TCP Dup ACK 41#1] 49736 > ftp [ACK] Seq=82 Ack=294 Win=65047 Len=0

Frame 44: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     45 37.274734   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 45: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     46 37.275388   TCP      62     [TCP Dup ACK 41#2] 49736 > ftp [ACK] Seq=82 Ack=294 Win=65047 Len=0

Frame 46: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     47 38.515559   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 47: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     48 38.516182   TCP      62     [TCP Dup ACK 41#3] 49736 > ftp [ACK] Seq=82 Ack=294 Win=65047 Len=0

Frame 48: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     49 40.376003   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 49: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     50 40.376567   TCP      62     [TCP Dup ACK 41#4] 49736 > ftp [ACK] Seq=82 Ack=294 Win=65047 Len=0

Frame 50: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     51 43.474007   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 51: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     52 43.474611   TCP      62     [TCP Dup ACK 41#5] 49736 > ftp [ACK] Seq=82 Ack=294 Win=65047 Len=0

Frame 52: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     53 49.053924   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 53: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     54 49.054515   TCP      62     [TCP Dup ACK 41#6] 49736 > ftp [ACK] Seq=82 Ack=294 Win=65047 Len=0

Frame 54: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     55 59.593195   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 55: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 345, Ack: 82, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     56 59.593832   TCP      62     [TCP Dup ACK 41#7] 49736 > ftp [ACK] Seq=82 Ack=294 Win=65047 Len=0

Frame 56: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     57 66.796925   FTP      70     Request: TYPE A

Frame 57: 70 bytes on wire (560 bits), 70 bytes captured (560 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 82, Ack: 294, Len: 8
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     58 66.833363   TCP      62     ftp > 49736 [ACK] Seq=375 Ack=90 Win=32679 Len=0

Frame 58: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 375, Ack: 90, Len: 0

No.     Time        Protocol Length Info
     59 66.852889   FTP      92     Response: 200 Switching to ASCII mode.

Frame 59: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 375, Ack: 90, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     60 66.853447   TCP      62     [TCP Dup ACK 57#1] 49736 > ftp [ACK] Seq=90 Ack=294 Win=65047 Len=0

Frame 60: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 90, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     61 67.653089   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 61: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 375, Ack: 90, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     62 67.653593   TCP      62     [TCP Dup ACK 57#2] 49736 > ftp [ACK] Seq=90 Ack=294 Win=65047 Len=0

Frame 62: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 90, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     63 68.894847   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 63: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 375, Ack: 90, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     64 68.895352   TCP      62     [TCP Dup ACK 57#3] 49736 > ftp [ACK] Seq=90 Ack=294 Win=65047 Len=0

Frame 64: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 90, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     65 70.755267   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 65: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 375, Ack: 90, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     66 70.755790   TCP      62     [TCP Dup ACK 57#4] 49736 > ftp [ACK] Seq=90 Ack=294 Win=65047 Len=0

Frame 66: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 90, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     67 73.851892   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 67: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 375, Ack: 90, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     68 73.852389   TCP      62     [TCP Dup ACK 57#5] 49736 > ftp [ACK] Seq=90 Ack=294 Win=65047 Len=0

Frame 68: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 90, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     69 79.432036   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 69: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 375, Ack: 90, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     70 79.432625   TCP      62     [TCP Dup ACK 57#6] 49736 > ftp [ACK] Seq=90 Ack=294 Win=65047 Len=0

Frame 70: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 90, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     71 87.075370   FTP      70     Request: TYPE A

Frame 71: 70 bytes on wire (560 bits), 70 bytes captured (560 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 90, Ack: 294, Len: 8
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     72 87.111901   TCP      62     ftp > 49736 [ACK] Seq=405 Ack=98 Win=32671 Len=0

Frame 72: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 405, Ack: 98, Len: 0

No.     Time        Protocol Length Info
     73 87.131366   FTP      92     Response: 200 Switching to ASCII mode.

Frame 73: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 405, Ack: 98, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     74 87.131840   TCP      62     [TCP Dup ACK 71#1] 49736 > ftp [ACK] Seq=98 Ack=294 Win=65047 Len=0

Frame 74: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 98, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     75 87.801062   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 75: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 405, Ack: 98, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     76 87.801573   TCP      62     [TCP Dup ACK 71#2] 49736 > ftp [ACK] Seq=98 Ack=294 Win=65047 Len=0

Frame 76: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 98, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     77 89.041426   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 77: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 405, Ack: 98, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     78 89.042081   TCP      62     [TCP Dup ACK 71#3] 49736 > ftp [ACK] Seq=98 Ack=294 Win=65047 Len=0

Frame 78: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 98, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     79 90.901416   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 79: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 405, Ack: 98, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     80 90.901988   TCP      62     [TCP Dup ACK 71#4] 49736 > ftp [ACK] Seq=98 Ack=294 Win=65047 Len=0

Frame 80: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 98, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     81 94.000587   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 81: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 405, Ack: 98, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     82 94.001269   TCP      62     [TCP Dup ACK 71#5] 49736 > ftp [ACK] Seq=98 Ack=294 Win=65047 Len=0

Frame 82: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 98, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     83 99.580475   FTP      92     [TCP Retransmission] Response: 200 Switching to ASCII mode.

Frame 83: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 405, Ack: 98, Len: 30
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     84 99.581110   TCP      62     [TCP Dup ACK 71#6] 49736 > ftp [ACK] Seq=98 Ack=294 Win=65047 Len=0

Frame 84: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 98, Ack: 294, Len: 0

No.     Time        Protocol Length Info
     85 117.494304  FTP      70     Request: TYPE A

Frame 85: 70 bytes on wire (560 bits), 70 bytes captured (560 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: 49736 (49736), Dst Port: ftp (21), Seq: 98, Ack: 294, Len: 8
File Transfer Protocol (FTP)

No.     Time        Protocol Length Info
     86 117.531119  TCP      62     ftp > 49736 [RST] Seq=294 Win=0 Len=0

Frame 86: 62 bytes on wire (496 bits), 62 bytes captured (496 bits)
Ethernet II
PPP-over-Ethernet Session
Point-to-Point Protocol
Internet Protocol Version 4
Transmission Control Protocol, Src Port: ftp (21), Dst Port: 49736 (49736), Seq: 294, Len: 0

edit

I checked with another FTP server that is under my control and stuff get's weird now (read: it's not working with that one either). According to my server, it is communicating with an IPv4 address ending on .0. For the record, I'm located in Germany. I've never seen an IPv4 address ending on .0 before.

If I check the router on the client side it reports a completely different public IPv4 address (at least that one looks valid) and it also reports a IPv6 address (extremely seldom in Germany to have one. Depends on the ISP.)

And my server's logs are saying that it is sending (and also retransmitting, as it's not getting an ACK) the Entering passive mode message. Which means, to me at least, that the ISP is loosing/dropping that package.

Should I contact the ISP with that information?

The ISP just contacted me. It was a configuration fault inside their network leading to the PASV package being dropped.