"Legacy mode" was switched off in my case. I use Android 10 by the way. So all in all this does not seem to by the problem in my case, does it?
I have found another nuance of my problem: When I create a shortcut to a playlist on my android home screen (via widget), it would work for a while. But after 1 day or so tapping on the playlist widget would open Poweramp only but not start the playlist. However, when I open the same playlist from within Poweramp, it would start correctly. Thus, I get the impression that Poweramp loses connectivity to my songs and that somehow it regains it, which is why the playlist within Poweramp would continue working. But the shortcut on my home screen would keep the "lost connectivity", which is why it would not work.
Still, there are times where even my playlists within Poweramp get corrupt and stop working (as mentioned in my first post).
All in all, this makes me quite sad because I like Poweramp, I have got used to it but need to rely on my music software...