VPN on OS X Lion Server
I was getting the same error. In my case it turned out to be a conflict with "Back to My Mac"—specifically within my Airport Extreme. Once I removed my iCloud user account from the Airport Extreme, OS X Server's VPN started responding.

If there are any Airport Extreme/Express devices on your network, definitely remove any iCloud/MobileMe accounts using Airport Utility.

You may also have Back to My Mac turned on in System Preferences—and it's conceivable that could cause a problem as well...

I recommend (at least temporarily) checking that no user accounts have Back to My Mac enabled in the iCloud pane of System Preferences; I would verify this on every user account on every Mac on your network.

Good luck!