Challenge
Backup of Remote Desktop Server VMs with Application-Aware Processing may fail with an Unknown error, error code 0x80131500. You will see error messages similar to:
"Failed to call RPC function 'Vss.GetSqlInfoForLastSnapshot2': Error code: 0x80131500. Failed to invoke func [GetSqlInfoForLastSnapshot2]: Unknown error 0x80131500. Failed to invoke func [RunProgramInGuest]: Unknown error 0x80131500",
or
"Failed to call RPC function 'Vss.FinishSnapshot': Error code: 0x80131500. Failed to invoke func [FinishSnapshot]: Unknown error 0x80131500. Failed to invoke func [RunProgramInGuest]: Unknown error 0x80131500".
Variations of the function name are possible, too.
Cause
Backup of RDS VMs with UPD using Application-Aware Processing via PowerShell Direct on Hyper-V will make Veeam log into a temporary user profile. This will cause backup to fail with Unknown error, error code 0x80131500.
Backup will not fail if the user account utilized for guest processing has an active session on the RDS server at the time the backup is being performed. In this case the user profile disk is mounted and Veeam is not forced into temporary user profile.
Solution
Currently there is no solution. You can still back up Remote Desktop Server VMs with Application-Aware Processing over network, or use Hyper-V Native Quiescence for application consistent backups.