Forum Discussion
Outlook sync errors with Gmail and IMAP
Just to update on this, I have resolved my own issue.
I deleted my mail profile, uninstalled office, deleted all the credentials I'd used from Windows, deleted the Office folder entirely from AppData (Local & Roaming), then went through the registry and deleted anything that related to Office, and used CCleaner to clean it up afterwards.
Then I rebooted and reinstalled and finally everything works again. What was different this time was Outlook wasn't aware of my accounts at all, and the setup process was entirely different. No longer does it use App passwords, it just pops up a box from Google to authenticate. This seems like it may have been the issue, so I assume Gmail accounts set up with app passwords will fail in some Office updates until they are deleted from the Windows credential manager (or possibly more extreme doing as I did above).