remove ozone from plugins dir, restart Renoise, do a rescan, close Renoise, add ozone back to plug-ins dir, start renoise and do a rescan.
I didn’t do that because it seemed tedious : ), but while looking at the AppData\Roaming Renoise folder I saw there were assorted SQLite database files for cached VSTs, including failures. I moved them out of the folder and loaded the troublesome song. Renoise rescanned all the VST folders.
I got an error about an Ozone 9 Vintage EQ VST failing to load, and it said it would not load again until I removed that cache db. Sort of weird since that’s not the name of the plugin I was using (there is a vintage EQ plugin but I was using the regular one), but whatever. I removed the db files again, reloaded the song, and everything comes up fine.
I wish I took screenshots of the error messages because my recounting here might not be exact. The upside though is that if you have missing VSTs in a song, go find those db files, delete them (they get rebuilt), and try loading the song again.
This seems to have fixed the problem.
Thank you everyone for your comments and suggestions!