Finally checking again all connections, I had a cold solder on RX.
After I had corrected this and soldered it again, I was able to flash it again and now everything seems to be fine again.
Will put it into my quad and see if everything is working as expected.
Thanks to everybody for the help, hints and advises.
Everything is working, I just made a short test in my garden, no flight, just checking the rotation of all motors and that it reacts correctly to stick input.
Tare is done, next I will do the onboard magnetic calibration.
And then I will have to calibrate my AlexMos Gimbal and the maiden flight will be one step away.