It´s a key related of course - cycling through the voices makes same effect on that key. Always high like aftertouch is maxed all the time...
I swapped the IC and it changes nothing. Soldering seems ok. Contact is also looking good.
I traced the signal to the IC4 and it´s there. No idea where it should come out before going to TKC tough. Could be one of the new caps on the way - I don´t know.