It looks like I have found the bug! 
after the factory reset, I set everything up again one by one and checked the camera after each step.
I had created a tile for quick settings, a button to turn on and off the sensors.
Of course I turned these off and didn’t expect the camera needs them though.
The microphone needs it unfortunately and therefore I put it in a tile to be able to switch faster.
So every time I turn off the sensors, I get the error from the camera.
I am very glad to have found this out.
Thanks a million for your kind help and inspiration.
Maybe this conversation will help others too.
Kind greetings, Silver