I have an answer, but it has taken a long time to figure it out.
I have a Mac Pro, running behind an Airport Extreme 811N router. I ran OSX Server 10.6x and after I did the upgrade to 7.5.x firmware on my airport the L2PT service died going thru my router. I simply switched to the PPTP VPN because it appeared to work fine. Then I upgraded (or downgraded) to 10.7x Server. When I did that they got rid of PPTP as an option, and my L2PT connections still did not work. I went looking online for answers, and found alot of references to the 7.5.x firmware. I ran a test to see if I could connect to the VPN internal to the LAN - thereby bypassing the router as an issue. It worked flawlessly. It definately had something to do with the way 7.5.x handles a packet.
After several trial/error sessions, I figured out that it was the DCHP service on the Airport Extreme that was causing the problem. For whatever reason if you have DCHP assign the IP address to your VPN server, it will never work. I took the server out of the DCHP pool, and gave it a static IP. Once I did that and correctly configured the interface on my server (be sure to setup the DNS correctly if you use static IP) I was able to get the VPN to work flawlessly. Was even able to turn the Back to my Mac feature back on.
Don't know if this helps, but I have personally logged 3 days on this problem over the last 2 months. I am pleased it is resolved.