For better understanding;
Are you using the NUC’s power adapter that came with the NUC?
Does it occur with other devices?
Yes. I am using the NUC's power adapter.
As I mentioned I did all kind of tests and I manage to find out that it is related to the NUC.
Other computers in the same room which use same screen don't have such problem.
This NUC has problem also with another screen and also in another room - So I think it is safe to say that is NUC issue.
You might want to try adding an EMI filter...