Looks like it's related to this bug - https://forums.plex.tv/t/library-db-size-more-than-doubled-in-latest-version/918851 I ran the DBRepair script from ChuchPa and it pruned my bloated DB from 21GB to 250MB and now plex is all good again.
I'm having the same issue, visiting the WebUI gives me this - <Response code="503" title="Maintenance" status="Plex Media Server is currently running database migrations."/> Guess I'll just wait, I have backups but not sure if I should restore or not.