I've sent it to dealer, since I bought it in Thomann.de and still have it under warranty.
They've fixed it in no time and now I've got it back fully working.
They've reported this:
- reconnected the losing ribbon cable; recalibrated the pitchwheel, modulation wheel & osc/filt; performed a visual inspection, VDE-safety check & functional check = issue resolved/staff1338
So I'm not really sure, what was the problem, but all is good now. :-)