This seems a X server reset at the login screen that is not caught by the node.
Explanation: After the login, the X server resets and closes all the connections by clients, including the NoMachine node process. Then the X server begin listening again, so the desktop environment can start. The node should try to connect again automatically but it is mistaking the reset.
Is this explanation correctly describing the problem? Can you please check if before and after the login screen the X server process id is still the same? You can do that using the ps command.
On the server do:
> ps auxwww|grep X
Then login. After the client disconnects, do again:
> ps auxwww|grep X
See if the pid of the X server process has changed.