I have an old external driver (Microstep R60 + Stepper) with which I have also tested the whole setup. When I connect the system to the power, the shaft of the motor is fixed by the holding current. If I change the R value, the system behaves the other way around. So just like with the integrated servo motor.
I conclude that it is not the servo but my wiring/expansion board.
This is the Result of the M122 command. In my perspective it looks ok.
M122 B9
Diagnostics for board 9:
Duet EXP1XD firmware version 3.2.2 (2021-02-12)
Bootloader ID: SAMC21 bootloader version 2.3 (2021-01-26b1)
Never used RAM 5932, free system stack 94 words
HEAT 90 CanAsync 89 CanRecv 83 MAIN 397 AIN 64
Last reset 00:09:25 ago, cause: software
Last software reset data not available
Driver 0: position 0, 80.0 steps/mm
Moves scheduled 0, completed 0, in progress 0, hiccups 0
No step interrupt scheduled
VIN: 24.4V
MCU temperature: min 29.8C, current 30.4C, max 30.6C
Ticks since heat task active 130, ADC conversions started 282561, completed 282561, timed out 0
Last sensors broadcast 0x00000000 found 0 134 ticks ago, loop time 0
CAN messages queued 18, send timeouts 0, received 2838, lost 0, free buffers 36