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.