SharePoint Online or OneDrive for Business backup jobs fail with “The remote server returned an error” warning

Challenge

Backup jobs with SharePoint Online or OneDrive for Business items fail with the following errors:

Error: The remote server returned an error: (500) Internal Server Error.
Error: The remote server returned an error: (409) Conflict.

NOTE:

• If you are experiencing these errors in versions higher than Veeam Backup for Microsoft Office 2.0, contact Technical Support.
• Different error codes starting from version 3.0 my appear. For more information, see this KB article.

Cause

Items being backed-up might be blocked by the internal SharePoint Online antivirus. For more information, see this Microsoft article.

Solution

To check whether items are infected, open OneDrive or Site which cannot be backed-up and try to download files manually. If the files you are trying to download are infected, the following error appears.