Avamar Hyper-V Incremental RCT backup fail with "Failed to get RCT extents for the disk file" or "Reference point creation for Virtual machine failed"

Avamar Hyper-V Incremental RCT backup is failing for some VMs with the error "Failed to get RCT extents for the disk file"

Here is a snippet from the backup log:

2019-07-08 23:44:41 avhypervvss Error <43426>: Checkpoint creation for Virtual machine '<VM_NAME>'<VM_GUID>' has failed and will be skipped from backup. For details, please check 'winclustersvc.log'.
2019-07-08 23:44:41 avhypervvss Error <43940>: Failed to get the RCT extents for Virtual machine '<VM_NAME>'<VM_GUID>' and will be skipped from backup. For details, please check 'winclustersvc.log'.

From the winclustersvc.log file (located in the Avamar VAR folder on the Primary Hyper-V Node):

2019-07-08 23:44:35 winclustersvc ERROR<0000>: Failed to get RCT extents for the disk file C:\ClusterStorage\<VM_PATH_TO_VHDX>: Failed to get the disk information.

Also, the backup could fail with the error below:

"Reference point creation for Virtual machine '<VM_NAME>'<VM_GUID>' has failed".

One of the cluster nodes hosts the affected VMs.

There is no Warning or Error message in the Windows Application System, or the Hyper-V Event Viewer Logs.
The "Reference point creation for Virtual machine" error is only seen in Avamar Logs. 

Here is the relevant portion from winclustersvc.log:

2019-06-25 18:19:41 winclustersvc DEBUG<0000>: Affected VM Path is \\<Hyperv_Node>\root\virtualization\v2:Msvm_ComputerSystem.CreationClassName="Msvm_ComputerSystem",Name="<.....>" 
2019-06-25 18:19:41 winclustersvc DEBUG<0000>: ===> ValidateJobOutput  
2019-06-25 18:27:09 winclustersvc DEBUG<0000>: <=== ValidateJobOutput  
2019-06-25 18:27:09 winclustersvc ERROR<0000>: Exception in validateTargetMergeStatus: Not found . 
2019-06-25 18:27:10 winclustersvc ERROR<0000>: Exception in validateTargetMergeStatus: Not found . 
2019-06-25 18:27:10 winclustersvc DEBUG<0000>: ResilientChangeTrackingIdentifiers rctid :rctX:917dbfe9:0da5:4fda:8d9f:34cdc6cf85b8:00000002 
2019-06-25 18:27:10 winclustersvc DEBUG<0000>: VirtualDiskIdentifiers vmid :Microsoft:<................>
2019-06-25 18:27:10 winclustersvc DEBUG<0000>: Affected VM Path is \\<Hyperv_Node>\root\virtualization\v2:Msvm_ComputerSystem.CreationClassName="Msvm_ComputerSystem",Name="<......>" 
2019-06-25 18:27:10 winclustersvc DEBUG<0000>: ===> ValidateJobOutput  
2019-06-25 18:27:10 winclustersvc DEBUG<0000>: <=== ValidateJobOutput  
2019-06-25 18:27:10 winclustersvc INFO<0000>:  Exception in reference point creation for vm group 'AvamarBackup_<........>' : Not found  
2019-06-25 18:27:10 winclustersvc INFO<0000>:  Reference point conversion failed for vm <VM_NAME>..
.
.
2019-06-25 18:27:10 winclustersvc DEBUG<0000>: ===> SerializeHypervVMList. 
2019-06-25 18:27:10 winclustersvc DEBUG<0000>: <=== SerializeHypervVMList. 
2019-06-25 18:27:10 winclustersvc INFO<0000>:  ----------------------End [EMC Avamar Plug-in Service for Windows] log ------------------- 
2019-06-25 20:15:08 winclustersvc INFO<0000>:  Cleaning up checkpoint created on group AvamarBackup_<........>
2019-06-25 20:15:08 winclustersvc WARN<0000>:  Failed to execute DestroySnapshot method: The method call failed.. 
2019-06-25 20:15:08 winclustersvc INFO<0000>:  Cleaning up checkpoint created on group AvamarBackup_<........>
2019-06-25 20:15:08 winclustersvc WARN<0000>:  Failed to execute DestroySnapshot method: The method call failed.. 
2019-06-25 21:31:32 winclustersvc INFO<0000>:  Creating an alternate Log file : C:\ClusterStorage\<...>\var-share\temp_log.log 
2019-06-25 21:31:32 winclustersvc INFO<0000>:  Debug output enabled from Plug-in. 

Every time the backup is forced to full and failed at the end due to 'reference point error', so the virtual machine (VM) gets excluded.

Hyper-V VSS plug-in originally used the Hyper-V namespace in WMI calls to get the RCT extents. For the new hotfixes, the code has been changed to reference cluster namespace instead.

The Avamar Hyper-V cumulative hotfixes are required which include the fix for this issue.
They incorporate the latest code changes for Resilient Change Tracking (RCT) for both backup and restore improvements.

List of Hyper-V plug-in's latest cumulative hotfixes:

• Avamar Plugin version 19.9.100-78 HF 338821
• Avamar Plugin version 19.10.100-135 HF 338820
• Avamar Plugin version 19.10.100-166 HF not required 

To download the hotfixes, see the section "Looking for a Hotfix knowing the HF number" of the below article:

• Avamar: How to find and download a product hotfix, patch, install or upgrade package from the Dell Support website

For instructions on applying the hotfixes or upgrading to the latest Avamar version, use the following DELL KB article:

• Avamar: Hyper-V VSS plug-in - Configuration and installation of latest cumulative hotfixes for version 19.4 and higher