Info post only, in case others have this error in the future.
I was stumped. My Nextcloud install was stuck in an upgrade loop. The landing page said
App update required
The following apps will be updated:
- File sharing (files_sharing)
Please make sure that the database, the config folder and the data folder have been backed up before proceeding.
There was a big shiny blue button: Start Update. I hit the button, it seemed to be updating, but then eventually returned to the same screen.
Eventually in the log (search in your file interface for nextcloud.log) I found a message a ways back (not logged as an error, mind you, just debug level):
appinfo/database.xml file is not longer supported. Used in files_sharing
With that I was able to find a reddit post with instructions that worked for me: https://www.reddit.com/r/NextCloud/comments/qd1k8o/appinfodatabasexml_no_longer_supported_after/