VPN to a Kubernetes-cluster from a remote network

Ta-damn!

pfSense was breaking the SYN-packet's checksum:

13:53:32.286601 IP (tos 0x0, ttl 62, id 33830, offset 0, flags [DF], proto TCP (6), length 60)
    10.103.103.1.47390 > 10.233.10.101.1337: Flags [S], cksum 0x86e4 (incorrect -> 0x99db), seq 4230752647, win 29200, options [mss 1460,sackOK,TS val 598846881 ecr 0,nop,wscale 7], length 0
        0x0000:  4500 003c 8426 4000 3e06 31e0 0a67 6701  E..<.&@.>.1..gg.
        0x0010:  0ae9 0a65 b91e 0539 fc2c 2987 0000 0000  ...e...9.,).....
        0x0020:  a002 7210 86e4 0000 0204 05b4 0402 080a  ..r.............
        0x0030:  23b1 ada1 0000 0000 0103 0307            #...........

I've disabled the hardware checksum offload feature and now everything works smoothly.

Lots of thanks to y'all for your time and attention!