False alarm!
My part cooling fan cable simply had a cable rupture.
The behaviour was strange anyway, the cable had contact and let the fan work on the idle and pause positions, but not in the middle of the bed (where the prints happen). A little bend of the cable strain and the part cooling fan worked again. Changed the cable with a spare one, works like a charm!
Thank you if you already spend time to my problem.
Happy printing!
Patrick