SharePoint Online or OneDrive for Business backup jobs fail with one of the following errors:
Error: The remote server returned an error: (500) Internal Server Error. Error: The remote server returned an error: (409) Conflict.
Files mentioned in the error stack are blocked by the internal SharePoint Online antivirus. If you download any of the affected files over the Office 365 web interface, you will receive a warning that a virus has been detected. The list of affected files you can find in your Veeam Backup for Microsoft Office 365 logs.
The corresponding errors and warnings will continue to appear in Veeam Backup for Microsoft Office 365, as long as affected files are blocked by the internal SharePoint Online antivirus.
For more details, please check this Microsoft article.