If you have any tasks which have completed with a warning you will see the number of warnings under the Backup Statuses chart in the Protected Intelligence dashboard located at https://backup.protectedintelligence.com
To view which users and files are affected go to Audit-> Tasks and use the filter for warning status.
You can then view the tasks to analyse which files are causing the warnings.
-The document is corrupted and can't be retrieved through the Office 365 API. In this case, we suggest trying 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 the corrupted file in OneDrive with a correct copy from one of the previous backups.
-The file is classified by Microsoft as malware and is prohibited from being downloaded via Office 365 API.
-OneNote documents can't be backed up if the user's document library contains too many items or there are too many OneNote notebooks/sections/section groups inside the 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.