Hi andrea,
the original problem looks like a problem with video drivers.
May I assume you are using NoMachine free on server side and you are connecting to the physical desktop of that machine?
Can you check the CPU usage on server side and tell us what processes, if any, consume most of it?
Logs could also be useful. You can find instructions for retrieving them in https://www.nomachine.com/AR07K00677. You can attach logs here or send to forum[at]nomachine[dot]com.
Another possibility could be bad network conditions. You could try to lower the Display quality using the slider in the Display settings tab and check if this alleviates the problem in any way.