I recently looked into replacing my Netgear VPN router with a Linux machine. I have configured it and everything worked perfectly including port forwarding. Except that when I tried to connect to a web hosting company's PPTP server, it failed with error 619. The previous router has no such issue.
I tried to connect another PPTP server (running on Linux) and it connected successfully.
In the Linux router, I forwared 1723 and gre to an internal PPTP server. I thought that they are only needed for incoming PPTP connections not for outgoing..