Starting on December 11th, 2023, Exchange backup within Veeam Backup for Microsoft 365 fails with the following errors:
There is an error in XML document
The specified type was not recognized: name='SharingMessage'
Veeam has observed that there have been changes to the structure of the XML message received from M365 referencing the SharingMessageType property received from EWS. The Veeam Backup for Microsoft 365 software is unable to process this XML structure.