Symptom:

If you see error messages like the following in the Message Logs of the backup:

VSS Snapshot Creation failed. Check Log file for more details.
Warning: Plugin VssPlugin do_VSS failed.
Warning: Plugin VssPlugin Post VSS failed.
Running as 'SYSTEM'. Privmask=00000206
VSS PLUGIN start backup job
Warning: Plugin VssPlugin finish_VSS failed.

Cause:

The VSS failure you are seeing on the backup is due to insufficient storage space available to create the VSS snapshot.

The actual error text in the trace files is: "Error text: VSS_E_INSUFFICIENT_STORAGE"

Solution:

Run systeminfo from Diagnostics and you can see the available resources for the server.

If this is a Virtual Machine (VM), you may be able to devote more system resources to that server and get away from the error.

See [Microsoft Docs](https://docs.microsoft.com/en-us/windows/desktop/api/vsbackup/nf-vsbackup-ivssbackupcomponents-dosnapshotset){:target=\_blank} for further details.

-it suggests looking for and if possible, deleting old shadow copies.