(Linked from Setup a VPN server with Mavericks Server 10.9)
So you wanted more than just a hint? Well, okay. As mentioned, the current version of Mavericks Server (3.0.1) has issues with L2TP VPN connectivity. Until Apple officially releases a fix for this, one way to work around this is to replace the copy of /usr/sbin/racoon shipping in Mavericks Server with a copy of /usr/sbin/racoon that shipped in Mountain Lion Server.
Below are nine command lines which will take care of this via the Terminal. If, in fact, you do want to try this workaround, paste these lines (each line below is an entirely separate line) into a Terminal window. You will likely be prompted for your password at least once.
The standard caveats apply: This is a temporary, unsupported workaround, and only a suggested idea at that. Again, it is entirely unsupported, even by Apple. Proceed with this workaround on your own equipment only at your own risk. And before making changes to your own server, remember the Golden Rule: always have backups of your data!
Paste these in terminal, one line at a time: