Avoid logging in to OS X Server (at the Login window) with a network user account whose home directory is hosted on the same server. If the issue has already occurred, restart the server.
When a network user logs out, the volume hosting the user's home directory is immediately unmounted. If a network user logs in on the server itself, the home directory volume is not unmounted until logout, preventing other users from accessing that volume until the server is restarted. This can result in users not being able to log in.