WIDWriter failed

We are using VEEAM Backup & Replication v9 to backup 2 X Windows server 2012r2 VMs on our VMWare 5.5 host.  The backup job has been running without a hitch for more than a year.  Over the last few days, the backup has been failing because one of the machines cannot be quiesced.  Checking the event logs on the VM shows a few errors as follows

Event ID 50:NTFS
Delayed Write Failed} Windows was unable to save all the data for the file . The data has been lost. This error may be caused by a failure of your computer hardware or network connection. Please try to save this file elsewhere.

Event ID 157:Disk
Disk 1 has been surprise removed.

Event ID 137:NTFS
The default transaction resource manager on volume \\?\Volume{0b861d7d-97fe-11e8-8117-000c29d9844f} encountered a non-retryable error and could not start.  The data contains the error code.

Event ID 58:Partmgr
The disk signature of disk 1 is equal to the disk signature of disk 0.


I have researched these errors and various sites and links have led me to look at the VSS writers, of which WIDSriter is in a non-retryable error state.  I have stopped relevant services, re-registered the writers and rebooted.  When I do this, WIDWriter is in a running state but after the next VEEAM backup attempt it reverts back to failed and the backup does not complete.

There is no Disk 1 attached to the machine so I can only surmise that this is being created on the fly during the Shadow Copy.

Any suggestions as to how to fix the WID writer as a first step?

Thanks