1477 - Exchange mailbox backup fails with error "Unexpected exception returned from the service"

Symptom

At a Redstor partner or customer using self-hosted storage, a backup of Exchange mailboxes fails with the following error:

Error: Code: generalException Message: Unexpected exception returned from the service. 

Recurrent 503 and 504 error codes are also seen in App Insights.

Cause

An HTTP 503 response (Service Unavailable) typically indicates a performance issue on the server, while HTTP 504 (Gateway Timeout) indicates that the server did not receive a response in time to complete the request.

Solution

To resolve this, restart the StorageServer service and run the backup again.

Note: This issue should not affect users on Redstor hosted and managed storage, as the operational and connectivity status of this is closely monitored.