@Niggich
What was the trick that made it work finally?

I noticed the following:
When using omxplayer i also had a black screen instead of the camera image.
I removed the sd card and inserted it in a different raspberry.
Then everything was working fine.
The only difference between those 2 is the monitor connected to it.
It was working on a 7 inch monitor and on a 24 inch monitor it didn’t work.
The problem was the resolution.
Now the module is using stream2 and i changed in my Hikvision camera the resolution settings for stream2.
Now my mirror is working fine.
Maybe this is also a solution for you.