See attached screenshot.
I was able to get tech support on the phone, who walked me through opening the file in repair mode. Although it didn't immediately work (was getting the same error,) after a couple of tries it seemed to all be fixed. I'm not sure if the repair solved it, as there were no errors found, or if it was just some weird fluke glitch.
Hopefully it doesn't happen again, or to anyone else, but if it does I guess the place to start troubleshooting is to open in repair mode.