There are instances when snapshots are fail due to an agent's VSS writers being in a failed state but it is impossible or not desirable to restart the server until at least after business hours.
Resolution:
An alternative solution other than restarting the server is to restart the associated services for each of the VSS writers showing up in a failed state by following the steps below:
Find each of the VSS writers in a failed state by using the command in command prompt (Run As Administrator) - ' vssadmin list writers '.
Mark and copy all the failed VSS writers.
Find the VSS writer's associated Service Display Name in the table below and restart the service.
To confirm the status has changed Stable with no Error - type - vssadmin list writers
Run another Backup/snapshot.
If the VSS writers fail again, it would be necessary to restart the server.