Yeah, but this is, after all, my best guess.
I think I got this issue when I downloaded Cervantes creature project models.
I have had some of his supermodels break many of the creatures …but I love lots of his work
What I’d do is cut the Gordian Knot instead of trying to unravel it.
Copy all custom content and saved characters and modules to backups.
Reinstall over the top of the existing game. (This shouldn’t destroy custom content, but it might, hence the previous step).
Check that it now works. If all of your custom content also now works, you’re done.
Restore custom content one subfolder at a time if anything was overwritten by the restore, retesting each time to see if anything breaks.
If it nothing breaks, you’re done.
If it does, you’ll know that what broke has to be in whatever subfolder you last restored.
Back-track and restore just one file at a time from that subfolder, testing each time until it breaks again, then you know that whichever file you restored caused the problem.