Now my sensor is working. It shows the right values. Here is my M591 D0:
m591 d0 Duet3D rotating magnet filament monitor v3 on pin 121.io1.in, enabled, sensitivity 25.77mm/rev, allow 90% to 110%, check every 3.0mm, version 3, mag 132 agc 83, measured sensitivity 25.12mm/rev, min 97% max 103% over 103.6mm
After changing the P value back to 3, my problems are solved. Great safety feature...
Thank you for all your help,