Gilwe,
I got several (!) replies. One o them is written by an engineer. He expects an IC (Integrated Circuit) to cause this problem. The IC, Ketron M01, must be taken out its socket, clean the contacts and re-insert it again. You probably better ask an electronic-engineer to fix this job for you ?

I got this instructions from info@ketron-musixx.de (since 1-1-02 the official distributor for Germany)

Tonight I will forward you the original mails.

Roel