I tested the device. The resistance between the contacts of the heating element was 150 Ohm, which should be ok, I think.. So I had no idea what caused the Printo not to heat up. The inside of the relatively simple electronics -only about 5 elements- looked ok to me. So I decided to put the thing back together.
Although I did not replace any parts, after reassembling the unit, it worked again ! The only thing that I did change a little was organizing the stainless steel wire that is connected to the heating element. Maybe it needs a little bit of attention once in a while..
regards