Having issues with the VPN service for OS X Server Mavericks not accepting L2TP connections?
According to Apple the problem can happen if the client and server are behind different network address translation (NAT) gateways.
To resolve the fault Apple has released an update to Mavericks v10.9.1.
The 1.6MB VPN Update for Mavericks Server requires OS X Mavericks 10.9.1 and Server 3.0.1 to run.
It will only install if you have already launched Server app and used it to set up services.
Via: Mac Observer.