Avamar: Hyper-V backup failing with hypervvss_assist DEBUG-INTERNAL ERROR

Hyper-V backup fails with the following error reported in the log:

[hypervvss_assist] INTERNAL ERROR: <0001> assert error (this_disk.virtual_disk_snapshot_id.isempty()), hypervvss_targets.cpp

This error occurs right after Avamar is fetching the virtual hard disk (VHD) list of one or several VMs:

avhypervvss Info <41738>: Fetching VHD chain list for 'VM_NAME<VM_ID>

Even if the avtar flag shown below is used, the backup ends with a "Failed" status instead of "Completed With Exceptions."

 --save-partial-backup

This issue is seen in a Full backup with a virtual machine (VM) creating on a differencing disk or checkpoint.

In such a case, this disk does not have a snapshot id associated with it. 

Here are the steps to reproduce the problem:

1. Perform first backup of a VM with a differencing disk or checkpoint.
2. The backup job succeeds
3. Add one more differencing disk with Hyper-V Manager.
4. Run backup.

Observed behavior: Backup fails.
Expected behavior: Backup should succeed.

Exclude the VM(s with the custom or user-created checkpoints. This can be done either using the dataset or using the "--excludelist" flag in the avhypervvss.cmd file (in the Avamar var folder).
Alternatively, remove the checkpoint from the VM. This can be done either using Hyper-V Manager or from the following PowerShell command:

Get-VMSnapshot -ComputerName "MyHyperVHost" -VMName "VM with Checkpoint” | Remove-VMSnapshot

If neither of the above workarounds is feasible, then contact DELL support and reference this KB article.

The "--save-partial-backup" flag was introduced to have an available Hyper-V backup to restore from, even if some VMs failed to backup.

This backup with some missing VMs ends as "Completed With Exceptions." The Avamar backup administrator should always review these exception errors.