I just tried something and I didn't think it would work but I used another GPIO for the step pin.. In wmh_focuser.cpp I changed it to:
#define M1_STEP_CHIP "gpiochip4"
#define M1_STEP_PIN 18
instead of 5.
So I'm using pin number 11 which has function1=PWM0 (see GPIO image on wiki.radxa.com/Rockpi4/hardware/gpio
).
It works like a charm I'm so happy
Thank's a lot for your help!