This problem occurs with NoMachine Enterprise Terminal Server Node v8.6 or later. When connection to physical desktop is enabled, the 'nxserver --daemon' process suddenly terminates while searching for the local display.

As a workaround, if connections to the physical desktop are not needed, remove type 'physical-desktop' from the AvailableSessionTypes key in server.cfg and node.cfg of the affected node.

Then, on the main server host (ETS) synchronize information by executing the 'nxserver --nodereadd NODE:PORT' command where NODE:PORT is the name of the node as it appears in the output of the 'nxserver --nodelist' command.

Note that the sudden termination of the process causes also the termination of the virtual desktops, which should instead stay running.