Hyper-V backups are failing with Error: The operation failed. (4042969)

Hyper-V VMs are failing to backup with error "The operation failed" when it is about to finish the Job.

-The Host Agents were upgraded to the current version.

-Specific VMs are failing independently of their Host. 

[An extract of the Task Log]

[Date Time]: This task uses a Hyper-V snapshot.
[Date Time]: Backing up virtual machine disks ...
[Date Time]: Backing up disk: C:\ClusterName\VolumeName\VM_Name\Virtual Hard Disks\VMName_ID-String.avhdx
[Date Time]: Using filter type Active Block Mapping™ for disk: VMName_ID-String.avhdx
[Date Time]: Filtering out content of pagefile for disk: VMName_ID-String.avhdx
[Date Time]: Error: The operation failed.
[Date Time]: Backing up disk 'C:\ClusterName\VolumeName\VM_Name\Virtual Hard Disks\VMName_ID-String.avhdx' completed.
[Date Time]: Error: The operation failed

This "The operation failed" error also appears in the Hyper-V's Event Viewer logs at the moment the checkpoint must be consolidated.

The chain of checkpoints continues to grow since they fail to consolidate.

The Hosts were upgraded to Server 2016, but some VMs are still in Configuration 5.0 and have not yet been upgraded to 8.0. Please note that not all VMs in 5.0 will have this error.

Before proceeding, please Note that we don't support Hyper-V or Powershell commands, review these steps as necessary. We recommend that you select your latest checkpoint in your less critical or smallest VM that is affected and Export it in order to have a copy as a backup.

In these following steps, we'll upgrade the VMs' Configuration Version and then consolidate the checkpoints using PowerShell.

1. Shutdown the VM.
2. Upgrade the configuration version Using Hyper-V Manager.
3. Review if it is ok to Discard Save State.
4. Now at version 8, you can power on the VM at this moment if you would like to.
5. In Hyper-V Manager start the disk consolidation. Note that you might not see checkpoints in the UI but they can still exist in the Cluster Storage.
6. Use this command to see the VM state and determine that a checkpoint is currently open on the VM:
   1. Get-VMSnapshot -VMName vmnamehere.domain.com -ComputerName hyper-v.domain.com| fl
7. Command to remove the checkpoint and merge the AVHDX file into its parent VHDX file.
   1. Get-VMSnapshot -VMName vmnamehere.domain.com -ComputerName hyper-v.domain.com| Remove-VMSnapshot
8. Hyper-V will now start processing the consolidation, please wait for it to complete and then test the backup job