When users connect to the physical desktop of a Linux headless host, a black screen is displayed in the NoMachine session.

This happens when the X server is up and running but the Linux machine doesn't have a monitory physically connected.

As a possible workaround, stop the X server or plug-in a HDMI Dummy Plug adapter or similar.

This problem affects those headless machines that were working with NoMachine versions previous than v. 6.7. For other cases, please consult:

https://www.nomachine.com/AR03P00973
How to solve black screen on headless machines