Improved logging helped a lot! Now, one month and a week later the bug has shown again and I have much more information to haunt the bug down. Bug is pretty much located, but not understood totally by me, I think (hope) it will be fixed tomorrow.
Discussion
Das war auch schon ein bisschen Arbeit bis hier her und das Ding ist noch nicht Mal gefixed 🙃
The bug hasn't shown up since more than a month. So it seems to be fixed. The logging will be keeped and if the bug shows up again, I have some utils to hunt it down.