vpn drops every 3 minutes exactly

[Guest]

[starwarp]

This only happens for 1 user which is using a dlink dir-615 router on the home side.  It did not happen when I had the netgear router -> windows server with port 1723 forwarded.

So on the endian firewall I have port 1723 and GRE forwarded to my windows RRAS server which handles the pptp connections.

I did a capture with tshark and came up with this last packet leaving my server in response to an echo request from the vpn client over pptp:
Broken TCP. The acknowledge field is nonzero while the ACK flag is not set

At exactly 3 minutes everytime that same message leaves my server and the vpn connection is dropped.

The other 3 users can stay connected forever no problem.