Manatus, your solution solved my problem. Thank you so much for being so clear, thorough, and helpful.

I had something like 12 gigs of data, mostly music, and had put off performing the remedy suggested in comment #11, since I'd have to restore data from backup. Today I re-checked this thread, saw this solution, tried it, and rebooted my phone. This time the "Updating library" screen only showed for less than a minute and now I can play all my music again! This will make exercising way more appealing again. :-)

Thanks again, Manatus.

P.S. Weirdly, the problem showed up just after I'd copied a bunch of sort of random files to my N9, like ZIP files and GPS waypoints, but deleting those immediately after noticing the tracker issue seemingly didn't fix it. Instead, once I looked at those error logs, I saw that I had to delete a few OGGs and MP3s that had been in a previous music-copying batch that had not seemed to initially cause any issues. Sorry I don't have time to look into this further & file bugs.