When you encounter an issue with just one VM failing, it’s almost certainly an issue inside the VM, not the host.
That’s because any VSS-aware service inside a VM can break the Hyper-V backup cycle by vetoing. When all VMs fail to back up, it’s likely something on the host causing the failure.
Please check the Event Viewer logs inside the VM at the time when the first backup error occurred, for example 1/11/2017 5:05:00 PM.
The time window of interest would be 1/11/2017 5:05:00 PM to 5:10PM, sometimes longer. Now look for VSS related errors in the Event Viewer logs. It could be any VSS aware service vetoing live backup, such as Exchange, AD, etc.
To rule out the possibility of a host issue, you can also run VssDiag and enter the exact time from above (1/11/2017 5:05:00 PM) in US format
VssDiag will collect Event Viewer errors on the host as shown here: http://backupchain.com/en/troubleshooting-volume-shadow-copy-errors-first-steps/
Of particular interest when you are dealing with Hyper-V are the logs in:
Applications and Services Logs -> Microsoft -> Windows -> Hyper-V XXXXXXX
as shown in the above article link.
Please contact our tech support for more information. We also offer remote assistance to get your backups running again asap.