When the NoMachine user disconnects, it has been noticed a sudden termination of the Cinnamon desktop that occurs when the popup 'desktop is viewed' is present. That termination doesn't occur when the popup is not shown, for example because it is disabled or the connected user is also the owner of the desktop.

As a workaround, don't show the 'desktop is viewed' popup: open the UI -> Settings -> Server -> Security -> section 'Desktop Notifications' and uncheck option 'Show that the desktop is viewed when somebody connects'.