@dc42 Thanks for your recommendations. I have run the M122 for the expansion board in question. See the screenshot attached. By comparing with the diagnostics from the main board, I believe this is looking ok.
ScreenshotM122_Exp1.PNG

I also metered all the body diodes. In reverse they are all blocking. In forward they all have about 540 mV, which I think is a reasonable value.

Since these results can't really explain whats wrong I did one more thing:

Taking a stepper that I know is working and reassuring this by quickly testing it on a stepper port that I know is working. It works. Connecting this stepper to one of the presumably broken stepper ports on the expansion board Connecting an oszilloscope first between DRIVER_0_A1 and DRIVER_0_A2 and second between DRIVER_0_B1 and DRIVER_0_B2. Unfortunately I don't have differential probes at home, so I had to take two separate measurements. See pictures attached.
The pictures are 10V/div and 20ms/div.
I assume the first one looks as expected, since there is some more or less clean switching between +Vcc and -Vcc. I use 24V. I believe the small oscillations are to be expected and partially due to my low quality measurement gear.
But on the second picture You can see that there is nothing happening on phase B.
This also explains why I can hear the motor switching very lightly but nothing moves.
The other two stepper ports of the expansion board show the same results.
So it appears that phase B is broken. I couldn't tell why, since on the schematic I see absolute no difference between the two phases.

Driver0_A.jpeg
Driver0_B.jpeg

Finally, I also connected the oszilloscope directly to the stepper drivers and measured between HB1-DRIVER_0_B1, HB2-DRIVER_0_B2 and LB1-GND, LB2-GND, so the gate signals to the MOSFETs. See pictures attached. This time it is 50mV/div. For HB1 and LB2 some 100mV is definetly to little to make a MOSFET switch. also the waveform looks uncommon. For HB2 and LB1 there is nearly nothing happening. This time I only measured Driver 0, since I had to solder tiny wires to the pins which is very annoying. I will assume the other two drivers will behave the same.

HB1.jpeg
HB2.jpeg
LB1.jpeg
LB2.jpeg

So in conclusion I definetly think I broke the stepper drivers. I can't tell if the MOSFETs are broken too. The body diode seems to be fine. I think this would have been the only thing to break the MOSFETs in a reverse voltage event.

I will try to get my hands on some replacement parts and post my experiences.
Thanks again for Your help.