Ok - problem solved! A new member of our makerspace brought his scope. Turns out, there's a little glitch in the documentation of possible Gcodes für the M452 command:
@dc42 I suggested the following correction of the at GCode Wiki:
M452 C"exp.heater3" F100 ; laser uses heater3 pin, PWM frequency 100Hz
instead of:
M452 C"exp.heater3" Q100 ; laser uses heater3 pin, PWM frequency 100Hz
That letter must have cost me 5 nights 
With this change I got to play with the PWM-frequency:
- If i set it too low, for example to 500Hz - the default if no F parameter is set, I got the problem described above.
- If i set it too high, for example the maximum of about 65kHz, i oversaturated my power supply and the laser kept burning at maximum power.
In the end i settled for 12500Hz - and lazing is fun again!!
