So managed to get it working again.
First uninstall was just via the regular uninstall, which leaves a LOT of files on the system. I reinstalled oDrive 323 from the main thread, and the 401 error persisted.
I then used RevoUninstaller to scrub everything except my sync folder, and reinstalled the 323 again. I then setup sync again by dragging the folders from oDrive Explorer and it works normal.
So something happened on my machine on 2nd July around 9pm that caused the persistant 401 errors. I found the time by using ‘sync failed’ to see when it all began. Event Viewer doesn’t have anything strange so I have no clue …