Error in Job Log :
V-79-57344-34070 - AOFO: Initialization failure on: "Microsoft Information Store". Advanced Open File Option used: Microsoft Volume Shadow Copy Service (VSS).
The Exchange writer or the Exchange Replica writer is missing or in an error state.
If the Exchange writer is missing:
Reboot the Exchange server and retry the backup job.
In a DAG environment :
If the Exchange Replica Writer has errored out, check the event viewer for the VSS errors on the respective Exchange Node.
Reboot the Exchange server and if problem persists, contact Microsoft support for further assistance.