Router – DD-WRT VPN error 800

Do not to use DHCP to assign an IP, and specify a range manually instead for the incoming connection. Thats what fixed it for me.

It's not entirely clear to me how you got PPTP working to the Windows XP server when you have forwarded all port 1723 traffic to the Debian Squeeze server. You likely can only "VPN" to the WinXP server from within your local LAN, which seems to be of limited usefulness.

Regardless, PPTP requires not only TCP port 1723 traffic, but also GRE protocol. Is your router capable of handling GRE tunnels correctly? If it's an ordinary consumer-grade router, then I suspect not. And even if it is, GRE is esoteric enough that finding help may be difficult.

In your case I recommend trying a VPN solution that uses only TCP and/or UDP transports, since those protocols are ubiquitous and well-known. OpenVPN is one such VPN solution, and it is available for all the major OSes (Win, Mac, Linux, *BSD).

Depending on what you're trying to accomplish, another possibility is to run sshd on the Debian server, e.g.:

apt-get install openssh-server

All the major OSes have free ssh clients capable of creating tunnels over an ssh connection.