So far....

The trick at the end was that I had to actually connect website and webspace with each other. And I had tried that. Many times. But that obviously didn’t work. I used some other way which did the trick eventually. The strange thing is that another website I am doing for a friend does not work. Actually the backend works, the frontend does not. The only reason I am able to imagine is that the error message on the frontend side is still in some cache of the website.