Windows Issues with L2TP VPNs
There is an issue with some windows systems behind some types of NAT. That is, when connecting to a Badjr PPTP works, L2TP on Mac works, but L2TP on Windows doesn't work.
This could be because of the issue details here. Even though the article refers to Windows Vista and Server 2008, it persists beyond those versions.
If you are happy to apply the patch mentioned in the article, you can use the registry file here