Error 619 when trying to connect with Windows VPN client

Solution 1:

I've finally found the holy grail:

http://www.jcsilva.com/2011/01/09/issues-with-dd-wrt-gre-forwarding-pptp/

This page says the following:

Issues With DD-WRT GRE Forwarding PPTP

A quick fix for DD-WRT not forwarding GRE PPTP packets.

Add the following commands to the startup commands (Administration->Commands):

/sbin/insmod xt_connmark
/sbin/insmod xt_mark
/sbin/insmod nf_conntrack_proto_gre
/sbin/insmod nf_conntrack_pptp
/sbin/insmod nf_nat_proto_gre
/sbin/insmod nf_nat_pptp

As a side note, I'm not sure why the Professional machine was able to connect correctly and at this point I don't really care now that I have a real solution that isn't enabling the DMZ.