Backup jobs with SharePoint Online or OneDrive for Business items fail with the following warning: "Processing OneDrive Username (https://orgname-my.sharepoint.com/personal/user_name) finished with warning: Failed to backup item: /personal/user_name/Documents/File.zip, The remote server returned an error: (400) Bad Request".
Items being backed-up might be blocked by the internal SharePoint Online antivirus. For more information, see this Microsoft article.
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.