Thanks @hendjosh
Thanks for your help. It works now. It seems to be a problem of all boards without SafetySwitch: ChibiOS: add BRD_SAFETYENABLE on all boards by tridge · Pull Request #20812 · ArduPilot/ardupilot · GitHub
Until then, it helps to activate the non-existing Safety Switch with the MP as you describe there: Using MatekL431 adapters for PWM and DShot - #2 by hendjosh