Heureka, I got it! It must be the "Shader Detail".
If you are able with your Hardware,
to slide it (Options > Video > Advanced Grapics > Shader-Details) up to (at least) high,
then you will surely be thankful for receiving a whole new game!

This is how the lightning on my PC shows the "pitch black" areas.
I had no idea, that here is light even necessary for some Computer configurations.
Exculpation to all the testers, I wasn't listening to.