Ah, I found what the problem was. The tech who installed the Kiwisix inadvertently cut the trace from IC2 in KLM-369 at pin 7 that prevented -5v signal in the MUX and that made the whole muxed EG signal to be wrong, so my hunch wasn't too far away :)
My Polysix is at last perfectly working now after months of work and this fix saved me from returning to check the Kiwisix. Beware of that mod for MIDI in KLM-369 if you get these kinds of errors.
Best regards to all the group.