If your Domain Controller is using Azure AD Connect, it may have recently updated itself and caused the SQL VSS Writer to start failing when your backups try to kick off a snapshot.
In the Application Log you may see something like this:
SQLVDI: Loc=SignalAbort. Desc=Client initiates abort. ErrorCode=(0). Process=1924. Thread=2788. Client. Instance=LOCALDB#DatabaseName. VD=Global\{RegistryID}1_SQLVDIMemoryName_0.
The fix:
Open up Programs and Features and ‘Repair’ the installation of Microsoft SQL Server 2012 Express LocalDB. Reboot and re-run your backup.
*This also appears to be responsible for recent failures to backups done using VEEAM as well. Same fix.*
For this fix I used a blog post here: http://www.tecfused.com/2016/09/veeam-backup-fails-vss-writer-errror-0x800423f4-azure-ad-connect/
They also have a script posted in the comments which might come in handy if your client has multiple servers to update or if you find the backups failing often and want a more long-term (though not permanent) solution.
