If the server is headless, then NoMachine will create a virtual display. We run the virtual desktop only if we can’t find any physical session running, so at a glance it seems something on the system.
This system worked flawlessly until last week
What changed last week? Did you update something? NoMachine, the OS? Or maybe change a setting or something?
What desktop environment is it?
The logs might show some hints as to what is happening. Please check the instructions in the following article, making sure you reproduce the problem before gathering the logs from the server. Submit them to forum[at]nomachine[dot]com making sure to reference the title of your topic in the email subject. Thanks!
https://kb.nomachine.com/DT07S00243