In VEEAM O365 Backup, I got the error:
:: Failed to process OneDrive: <Full Name> (https://xxxx-my.sharepoint.com/personal/<username>). The remote server returned an error: (401) Unauthorized.
Apparantly rights have not quite been set right on the onedrives over at this customer, so I added the service account that runs the back-up as site-admin using the script from Lieben.
Hope this helps you.