A stop-gap workaround for this error has been included with Veeam Backup for Microsoft 365 5d cumulative patch KB4235. This fix will cause the 'TeamsMessagesData' folder to be skipped if it fails to backup. When this skip occurs, a warning will be displayed in the job to notify the backup admin that data backup was skipped. Skipped data is excluded from backup and will not be restorable. All other mailbox data will still be backed up properly and restorable.
Possible long-term changes for the future are currently being researched. As new information becomes available, this KB article will be updated.
Due to Microsoft's planned deprecation of Exchange Web Services (EWS) backup for the TeamsMessagesData folder, which was used in previous versions of Veeam Backup for Microsoft 365, starting with Veeam Backup for Microsoft 365 6a, the backup of Microsoft Teams data will now be performed using Microsoft Graph Export API for Teams to backup public channel messages noteUser chats will continue to be protected using the old method, until EWS backup of the TeamsMessagesData folder is fully deprecated by Microsoft.. However, to access these APIs, end-users must fill out a form with Microsoft to request access. Review KB4322 for more details.