This problem occurs with NoMachine client on Windows, when the user opens two or more sessions, it doesn't matter if connected to the same server or not.

To reproduce it:

1) open 2 NoMachine windows
2) copy from one window and paste to the other
3) repeat 1) and 2) for a number of times
At a certain point, a client suddenly disconnects and close.

As a workaround, focus on the window of another application before switching to another NoMachine client window.

Otherwise, if copy&paste is not needed, disable it on the server host. You can do that from Settings -> Server -> Security, scroll down till 'Clipboard' section. As an alternative you can disable it manually as explained here: https://kb.nomachine.com/DT07S00233#22

The problem is due to a Windows exception raised due to the access to the clipboard by two different session processes.