Symptoms
On March 1, OneDrive backup failed with on of these errors:
"Error authenticating with resource"
"A configuration error: 'Microsoft OneDrive is not available"
"Service 'Microsoft OneDrive' is temporarily not available"
"Microsoft OneDrive is not available: Error: 400 GET {\"error\":{\"code\":\"AuthenticationError\",\"message\":\"Error authenticating with resource\"
Cause
There was an outage on Microsoft side on March 1, from 8 AM GMT to 9:10 AM GMT.
Solution
The issue was resolved by Microsoft; all backups are running now.