If you have a Lion server behind a NAT router (for example, an Airport Extreme or Time Capsule) that is running a VPN service you may have difficulties connecting to it with Windows 7 using L2TP despite the correct setup.
I won’t go into the deep dive on this now, but just a total quick tip. You need to change the encapsulation parameters on Windows 7. Do that by setting a registry key:
…add a new DWORD value: