@artem_zin: @aaverin thank you very much for the very promising suggestions.
I got it working. But in the end I cannot tell what caused / solved the problem. Additionally to your tips I did a fresh clone of the repo.
The repo contains a branch, that has a totally different codebase. I checked that one out a couple of days ago and then switched back to the original code base. I have a feeling, that this caused AS hickups. I'm glad it worked out finally. Had to spend half a day on that one 😌