An Apple KnowledgeBase article said, "This can happen if the client and server are behind different network address translation (NAT) gateways." Thursday's update fixes that.
The patch notes:
This update fixes an issue that could prevent L2TP VPN connections from working with OS X Server, as described in http://support.apple.com/kb/TS5313
This update requires OS X Mavericks 10.9.1 and Server 3.0.1.
Note: the update will only install if you have already launched Server app and used it to set up services.
It's a 1.6MB download from Apple's support site, but Mavericks Server users should find it in the Mac App Store, too.