Now I managed to get a screenshot of the short overlay in the title bar → ‘Music Library Sync’
This probably has to do with the setting in Metadata Recovery (see screenshots).
The problem with the thick line below the track list is now a thing of the past. It’s just strange that after a longer period of time it changes to a thin line by itself. That led me to the wrong conclusion.
PS: Unfortunately, my community account was classified as spam by the system and deleted along with my postings, so this is now the second attempt! I hope it holds now!
Fritz (from Austria)