At this point, I stopped using the debug version.
Nothing to report...
First I removed the ini and bak file as Redwing suggested.
The only irregular report I got followed after that:
01/06/2019 14:11:58 - end status check
01/06/2019 14:11:58 - save library settings
01/06/2019 14:11:58 - close playlists
01/06/2019 14:11:58 - close library
01/06/2019 14:11:58 - save db
01/06/2019 14:11:58 - save lyrics
01/06/2019 14:11:58 - export playlists <---
01/06/2019 14:13:53 - save lyrics
01/06/2019 14:13:53 - close library done
01/06/2019 14:13:53 - close podcast
01/06/2019 14:13:53 - close folder monitor
01/06/2019 14:13:53 - done
I don't know what 'export playlists' actually does. Does it export all playlists that have that option checked? Or only the playlists (with that option checked) that have changed?
In any case: I removed the option in many playlists and the problem didn't occur anymore (not knowing whether that's related or not).
After a few more openings/closings, I put the original files back.
And nothing was ever irregular.
Guess that's good news.
And at the same time a bit embarrassing: sorry for asking attention...
I really don't know what all those former problems were about then...
Yet thanks again for the feedback and suggestions.