All of a sudden my Exchange 2010 backup is failing. I am using BE 2010 R3 with the latest patches. AOFO is not enabled. Exchange 2010 is on Windows 2008 R2. The is the error:

Final error: 0xe0001904 - A failure occurred querying the Writer status.  Writer Name: Exchange Server, Writer ID: {76FE1AC4-15F7-4BCD-987E-8E1ACB462FB7}, Last error: The VSS Writer failed, but the operation can be retried (0x800423f3), State: Failed during prepare snapshot operation (8).
Final error category: Security Errors

For additional information regarding this error refer to link V-79-57344-6404

V-79-57344-6404 - AOFO: Initialization failure on: "\\Server name\Microsoft Information Store\Mailbox Database xxxxxxxxxx". Advanced Open File Option used: Microsoft Volume Shadow Copy Service (VSS).
Snapshot provider error (0xE0001904): A failure occurred querying the Writer status. Writer Name: Exchange Server, Writer ID: {76FE1AC4-15F7-4BCD-987E-8E1ACB462FB7}, Last error: The VSS Writer failed, but the operation can be retried (0x800423f3), State: Failed during prepare snapshot operation (8).
Check the Windows Event Viewer for details.
 

I have rebooted the Exchange server but the job still fails with same error. I have dismount/mount the mailbox database but still getting same error. I have restart the Exchange Information Store service but still getting error. It was working fine before. Any ideas?