Apple released a software update last Thursday for Mac OS X 10.9 Mavericks Server which fixes an issue preventing some users from accessing VPN services over L2TP connections.
Updating the VPN for Mavericks Server fixes an issue that could cause L2TP connections to be denied access to the OS X Server VPN service. Before the release of the latter, a number of users complained about this inconvenience. According to an Apple support document, the problem was due to the use of different NAT routers by the client and the server.
With the recent VPN update, you must now use version 10.9.1 of OS X Mavericks and the server 3.0.1 to connect. Apple noted that the update can only be installed on a machine that has already launched the server application and used it to set up the appropriate services.
The new VPN for Mavericks Server is 1.6 MB in size and can be downloaded via Software Update or from the Apple Support Downloads web page.