0x800423f4, The writer experienced a non-transient error. If the backup process is retried, the error is likely to reoccur
However, the reason is usually a service vetoing the backup.
If it’s the Hyper-V VSS Writer reporting this error, check all Hyper-V logs
There’s now a tool to help you do this called VssDiag
Also note that each VM has the capacity of break the backup process on the host; hence you may need to check each VM’s Event Viewer logs for errors, too.
As always, we recommend switching to BackupChain 😉