Onedrive - Business - In "Credentials Needed" hell

After using Office365 professional for a year, yesterday I became unable to sync new or changed files.

Two things occurred that could be part of the problem:

1. I logged into to portal.office.com as one of my colleagues to test permissions correctly.
2. I ran the routine MS Windows 10 updates.

I can sync-to-local any files that were already in Onedrive and/or a SharePoint site.
I can load new files to Onedrive via browser.

I cannot sync any new files...whether stored locally to push to onedrive  or put in onedrive via browser to then sync to local drive.

Opening the local file then saving it saves it locally but at the next sync it fails with the "Credentials needed" Sync Problem. Opening it from browser causes same outcome.


I am a highly knowledgeable computer user (Server 2012 is running in my basement), but this one  has me totally bamboozled.

I have done all the instructions here, twice.

https://support.office.com/en-sg/article/Fix-OneDrive-for-Business-sync-problems-e12c6a8b-4bbe-4391-9c23-1a52b55a1967?ui=en-US&rs=en-SG&ad=SG

I've done the manual cache/local store and browse cache clearing so many times it hurts.

I did one additional step, (not offered by MS above) by disconnecting and reconnecting my office365 account in one of the Office Apps.