Exchange Backup Fails With 'SOAP header Action was not understood'

Veeam Backup for Microsoft 365 Backup of an Exchange mailbox fails with the error:

Mail item data export failed. SOAP header Action was not understood.

This a known issue that Veeam Software is currently investigating alongside Microsoft Support.

This article will be updated as more information becomes available.

Update 2023-01-03: Microsoft has identified the issue and is working to deploy a solution.
Update 2023-01-23: Microsoft has confirmed that the fix has been fully deployed.

As the underlying issue has now been fully resolved and was an issue for which no user action must be taken to cause or resolve, this article will be auto-archived after 45 days to prevent future misunderstandings.