Failed to snapshot VM: Task to create VM snapshot failed with reason '(null)' when volume shadow copy is configured in the VM
NetVault Backup VMware plugin has no control of the internal actions of the VMware tools VSS operations. VMware plugin will only request to snapshot the VM and it's up to vmware tools VSS operation to create the snapshot. If the VSS failed to snapshot the VM for some reason, checking the event viewer - application and system logs will show the exact error message.
There are numerous VSS error messages that may show in the application or system logs. Depending on the event ID or error message, check the vmware knowledgebase or Microsoft technet to find resolution to the VSS error.
VMware KB to resolve: Volume Shadow Copy Service error
Microsoft KB to resolve: Event ID 12298 — Volume Shadow Copy Service Operations
In some cases, upgrading or re-installing the guest VMware tools straight away helps on resolving the "failed to snapshot" issue.