The NX protocol is a remote X protocol and software quite similar to VNC but over ssh.
While using it for a machine there was this strange behavior that the session was not working anymore and can't be reestablished. Reinstalling and configuring the nx server software didn't fix the problem.
In the end I fixed the problem by removing the .Xauthority files located in the users home directory. There where 3 files named .Xauthority, .Xauthority-c, .Xauthority-l. I removed all of them and the connection worked again.
Maybe some one is running into the same problem and this helps him find his way out. Beside the logging functionality of NX needs some more love by it's developers.