Pitchbend Sensitivity Slider changes Pitch while playing note?

[Saxifraga]

Hi. I got a problem with the 'Pitch Bend Sense' slider on a used System-8.
When I switch to a patch for example SYS8 engine and press a C4 I don't get C4 but B3 or D4. It's not exactly that note though.
Main frequency is 440Hz and Transpose is 0. I checked that.
When I now move the 'Pitch Bend Sense' slider once full up and down, the note changes up and down too but rests at the C4 the synth should play to begin with. Sadly now I can't use the Pitch Bender, because the slider always changes the notes I play up. The Pitchbender then adds to that unwanted offset. Any ideas how this can happen and how to get rid of this behaviour? The synth is totaly fine in any other regard.
The Pitch Bender unit has been changed though and may need "recalibration"?
I don't think that's the problem though, because the Display tells me that the slider produces values from 0.255, which is the expected range.
I have no idea how this behaviour is even possible.
thx in advance!