@daniel Still welcome advice on how to proceed before I try to add music again. Also, some artwork is still broken. It seems to not respond to either restarting server, clearing the image cache nor reinstalling Roon Remote (iOS). However, sporadically an album fixes itself?
E.g. Open Qobuz resisted fixing even with repeated manual re-identification (Edit/Identify). However, today the artwork is spontaneously there.
Another album just fixed itself when I manually re-identified it. Some artwork spontaneously re-identified with a different (wrong release) album cover. E.g. one out of 2 CDs in the box set spontaneously got the album cover associated with a 2019 release, whereas the other CD had simply kept the original 1998 artwork which matches the CD set I own:
Hopefully these detailed symptoms help you eliminate/get a lead on the type of unbalance/recovery that has been suffered.