Error 'Your authentication token to this content source has expired' in Files Connect OAuth

1. Verify that the App domain of the Office 365 app is exact as per the documentation
2. For per user authentication:
 

Salesforce Classic
 

a. Click your name. 
b. My Settings | Personal | Authentication Settings for External Systems

 
Lightning Experience
 

a. Click your avatar.
b. Settings | Authentication Settings for External Systems

 

3. Click "Del" to delete the external authentication for the system that is showing the error.

4. Initiate the OAuth flow again and review Manage Your Files Connect External Data Source Authentication Credentials.

5. If the issue persists, create a new SharePoint app (on the SharePoint side NOT the Salesforce side).
6. After Step 5 the existing Authentication Provider in Salesforce setup will need to be updated or recreated.
7. Any External Data Source in Salesforce using the existing Authentication Provider will need to be re- validated and synced, or, if a new Authentication provider has been created this will need to be specified in the existing External Data source.  NOTE: do not delete any existing External Data Sources in Salesforce as these may be customized for specific Sharepoint Integration points and be referenced in other Salesforce objects.

Similar steps can be used also when Named Principal authentication is used.  Here the Sharepoint admin is authenticating on behalf of all users.

When addressing the "Your authentication token to this content source has expired" using the Azure method, if additional authentication errors are incurred try using the alternate method of Authentication. Sharepoint online has throttling limitations that can kick in if users are trying to authenticate with a large site collection and the Alternate Method seems to bypass this limitation.