Recently we have begun seeing the “Failed to backup item: FILEPATH, Download request retry timeout exceeded” error appear in jobs backing up OneDrive. Recent changes on the Microsoft side have begun preventing OneNote notebooks from being able to be downloaded. This prevents Veeam from being able to request and download the file to backup.
We have an active case open with Veeam while they work with Microsoft to resolve this issue. We will update this post once Veeam and Microsoft have provided a public workaround or hotfix.