If a user never opened the Files tab of a team channel in Microsoft Teams, this tab is not linked to the Files document library in a corresponding SharePoint team site. In this case, Microsoft Graph request for such a document library fails with "404 - Not found" error.
Since recently, the error code in Microsoft Graph response has changed to "400 - Bad request - Value cannot be null. Parameter name: token". Veeam Backup for Microsoft 365 fails to interpret this unexpected response and cannot successfully continue backup of such teams.