NovaBACKUP BE v19.x and newer (2019-04-16)
If a particular VM cannot be backed up, but all other VMs can be backed up, the cause can usually be found within the VM. Often the Hyper-V VSS host cannot create a checkpoint (= shadow copy) for such a VM. You will then find a corresponding message in the Windows Event Viewer on the Hyper-V host and within the VM in the Event Viewer "Application" and/or "System". Please check the Event Viewer for such messages, determine the cause of the messages and correct it.
Here you can find the event viewer for the HyperV role on the Hyper-V host:
Open Windows Event Viewer.
Goto "Application and Services Logs" => "Microsoft" => "Windows" => "Hyper-V-VMMS" => "Admin"
For the backup of Hyper-V VMs NovaBACKUP BE requests a checkpoint from the Hyper-V host. If the Hyper-V host cannot create a checkpoint and make it available as a backup source, the backup terminates.