A20M Enlarged built room - No retraction on stepper motors

[Tilapia]

Hello community

I have successfully extended the Z axles of my A20M as well as all involved wires and did a Z limit modification in the firmware (see my earlier post). So far - so good, the machine is up and running.

But now a new problem with both extruders occcurs: Moving filament in and out with both extruders works perfectly, feeding filament during a print is also no problem at all. A simple object with color fading - no problem to print. But as soon as the extruders should temporarely retract filament within a print, both extruders only produce a snorring sound with no visible movement of the stepper motor axles. The resistance of the filament within the PFTE-hose is absolutely OK, and there is no clogging in the hotend.

As I had problems with long stepper motor wires with another printer in the past (induction effect obviously caused the electronics to fail), so I properly separated every single wire going from the mainboard to the extruders without any noticable effect. As a test, I connected stepper motors from other manufacturers to the end of the wires (no extruder mechanics attached, no load) and simulated a print, same effect - feeding filament ok, retrieving filament ok, but retraction only snorring sound but no movement on the motor axles. As a further test I temporarely changed the Vref from 0.9 volt to 1.1 volt on my TMC2208 stepper drivers but no positive effect at all. Before the modifications, the printer was working perfectly with retraction speed 45 mm/s and retraction distance 7.0 mm (solved the heavy stringing issues on my machine). I reduced the retraction speed temporarely to 35 mm/s - again no positive effect at all. Now I have ordered a fresh pair of 1.5 m stepper motor wires, but they will take some time to arrive at my place...

Do you have an idea how to solve this problem?

Regards
Hans