Right.
I fixed mine with a few hours manual work:
After trying a brand new test Apple ID and an OS reinstall and then FRESH OS reinstall (Catalina) I have finally narrowed the issue down to my legacy library files.
Somewhere along the line over the last 15+ years of my music library form iTunes on Windows XP to 7 and then many OS X and Mac OS versions later, something has corrupted. Even the Catalina auto-import created during installation from the %username%/Music/iTunes files brought the error back when I tried to sync again.
I concluded had to re-create and rebuild my library to resolve.
My main concerns were keeping the Play Counts, Rating and Playlists.
I realised that I was able to preserve these by exporting .m3u playlists of the 5 Star Ratings and batches of Play count values and My own Playlists.
(You can majorly speed this process up by assigning a hotkey to the "Export Playlist" from the Keyboard settings if on a Mac)
Then after creating a brand new empty library (alt+ Open Music) and pointing the media directory back to the iTunes Media folder you can start importing the playlists and using them to batch update the library metadata.
Any Personal Playlists
Ratings Playlists:
- 5 star
- 4 star
- 3 star
- 2 star
- 1 star
For the play count data you will need to use the free DougScripts Add or Subtract Play Count v3.0 to batch set these back to as close as you want to depending on how many chunks you exported them in.
e.g.
Play Count Playlists:
- 70-90
- 60-69
- 50-59
- 40-49
- etc...
This way you keep a vague similarity to your original play counts.
It may take you a few hours but it's probably the quickest way to replicate a non-corrupt copy of your library that will work with Genius Sync/Apple Music Sync.
Fingers crossed your legacy genius data is still attached to your account. -If not at least this way your Library will talk to Apple's servers again to pull down new Genius results.
The only drawback I noticed was my album art didn't pull through, but because I had a botched Apple Music cloud sync (everything uploaded was greyed out and not accessible) it now seems to have re-populated itself along with more accurate play count values. (Yay)
Hope that helps.