Error 619 when trying to connect with Windows VPN client
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.