Thank you all for your ideas!
I mounted the CNC head to overcome the printer menu and then could update from 1.5.x firmware.
But the issue with the bad response of the Y-motors stayed as before.
Using the CNC positioning controls I tested the motor drives and found an issue in the Y-line: controls worked inverse ???
At the end I found a very curious solution I can’t understand but it works:
I interchanged the Y and Z controller cables at BOTH ends and now the motors move correctly and also bed calibration runs smoothly. So the cable designation on the controller is X-Z-Y instead of X-Y-Z, but the motors are connected in the correct X-Y-Z order !
Previously I re-connected the cables several times in the right order and I’m sure to have them put-in completely. With about 40 years electronic experience, I’m pretty sure to to haven’t made any failure.
God knows …
But happily it works now. Now I have to calm down a bit and then I’m looking forward to get the whole unit working - but not today 
Best
Peter