In rare cases backup task can finish with a warning when it fails to download one or more items (files, emails) from Microsoft 365. The common reasons why it happens are the following:
- The document is corrupted and can't be retrieved through Office 365 API. In this case we suggest to try opening the file through OneDrive user interface, most certainly it will fail with a 500 Server Error. In this case you will need to replace corrupted file in OneDrive with a correct copy from one of the previous backup.
- The file is classified by Microsoft as a malware and is prohibited from being download via Office 365 API.
- OneNote documents can't be backed up if user's document library contains too many items or there are too many OneNote notebooks / sections / section groups inside user's drive (https://blogs.msdn.microsoft.com/onenotedev/2016/09/11/onenote-api-calls-fail-with-a-large-number-of-items-in-a-sharepoint-document-library/)
- OneNote section is protected by user password so it is locked from third-party applications.
Troubleshooting backup tasks with Warning status
If any of backup tasks have finished with warning status you will see a non-zero number of Warnings in Backup statuses chart in the Overview dashboard. To find affected users and files you need to go to Audit -> Tasks tab and filter out tasks with Warning status, then you can expand tasks one by one and review affected files: