Exchange Backups failing with "Failed to access mailbox" starting September 13th, 2023
Challenge
Starting on September 13, 2023, some customers using Veeam Backup for Microsoft 365 to back up Microsoft 365 exchange data may have backup jobs fail to process mailboxes with the error:
Failed to access mailbox.. An internal server error occurred. The operation failed.
Cause
Microsoft has confirmed that this issue is related to the incident they've documented in their Health Dashboard as EX675238, which caused some Microsoft Exchange Online environments to have issues with RESTAPI calls used to access the mailbox externally.
Solution
At this time, the Microsoft investigation into the issue is still ongoing. This article will be updated as further information becomes available. Please follow both the Microsoft Dashboard and this KB for the latest news.
Advised Actions
- Check the Microsoft 365 Service Health Dashboard for the EX675238 event to see if you are impacted.
- As a possible workaround, in Veeam Backup for Microsoft 365 increase the number of job retries or reduce the number of configured proxy worker threads. This may allow the job to be completed successfully; however, this isn’t a guaranteed workaround.
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