OK – my problem is solved. This had nothing to do with NoMachine NX. The graphics card in the machine just stops working when no monitor is attached, which can be ok for a desktop computer (although I find it difficult to think of a reasonable scenario where one would deattach the monitor for a longer time in a desktop environment…). I used this solution to build a dummy monitor plug with 3x 68 Ohm resitors and I have the X-Environment up and runnning!
