Error: Unfreeze error: [Backup job failed.
Cannot create a shadow copy of the volumes containing writer’s data.
A VSS critical writer has failed. Writer name: [SqlServerWriter]. Class ID: [{a65faa63-5ea8-4ebc-9dbd-a0c4db26912a}]. Instance ID: [{60e3e5dd-73fc-46d6-ab6b-6b8723df09e3}]. Writer’s state: [VSS_WS_FAILED_AT_PREPARE_SNAPSHOT]. Error code: [0x800423f4].]

I restarted the SQL Server VSS Writer ( Can be done during production as only used for backups ) and a retry still errored.

In the end I needed to reboot the server