problem doing 'child vm snapshot' backup (hot backup), VSS event id 521 error#:

Host Server, 2012 R2 Data Center (latest updates)

Virtual Machine, 2012 R2 Essentials (latest updates)

Problem occurs when attempting to do (either in WSB GUI or PowerShell) "child VM snapshot" backup and fails with Backup Event ID 521, "A Volume Shadow Copy Service operation failed. Please check "VSS" and "SPP" application event logs for more information.  Detailed error: The operation ended before completion."

JobType          : Backup
StartTime        : 11/24/2014 10:52 PM
EndTime          : 11/24/2014 10:52 PM
JobState         : Completed
CurrentOperation :
HResult          : 807800A1
DetailedHResult  : 8078001D

Integration Services are... integrated and the VM has the necessary components.  However, it just does not make the VSS snapshot, it sits for a few seconds and terminates with the above error and the following event logged:

- System
  - Provider
   [ Name]  Microsoft-Windows-Backup
   [ Guid]  {1DB28F2E-8F80-4027-8C5A-A11F7F10F62D}

   EventID 521
   Version 2
   Level 2
   Task 0
   Opcode 0
   Keywords 0x8000000000000000

  - TimeCreated
   [ SystemTime]  2014-11-25T04:52:40.567308700Z
   EventRecordID 4850
   Correlation

  - Execution
   [ ProcessID]  4848
   [ ThreadID]  5016
   Channel Application
   Computer thehostcomputername
  - Security
   [ UserID]  S-1-5-18
  - EventData

  BackupTime 2014-11-25T04:52:35.410000000Z
  ErrorCode 0x807800a1
  ErrorMessage %%2155348129

The following is the script I use to build the backup job:

# Windows Server Backup - PowerShell
#
# FileSpec for VM
#$VMFile = New-WBFileSpec -FileSpec 'F:\HyperVMs\VM\VM-001\vm001.vhdx'

#get VM Host Components
$VMHostComponent = Get-WBVirtualMachine | ?{$_.VMName -eq 'Host Component'}

#get VM object for backup
$VMTarget = Get-WBVirtualMachine | ?{$_.VMName -eq 'VM-001'}

#Set Backup Disk
$WBDisk = Get-WBDisk | ?{$_.DiskNumber -eq '4'}

#create backup target
$BackupTarget = New-WBBackupTarget -Disk $WBDisk -PreserveExistingBackups $true

#Create a policy
$WBPolicy = New-WBPolicy

#Set Schedule
Set-WBSchedule -Policy $WBPolicy -Schedule 06:00 #6am

#add backup target to policy
Add-WBBackupTarget -Policy $WBPolicy -Target $BackupTarget -Force

#add vms to policy
Add-WBVirtualMachine -Policy $WBPolicy -VirtualMachine $VMTarget
Add-WBVirtualMachine -Policy $WBPolicy -VirtualMachine $VMHostComponent

#adjust policy for VSS FullBackup
Set-WBVssBackupOptions -Policy $WBPolicy -VssFullBackup

#add filespec to policy
#Add-WBFileSpec -Policy $WBPolicy $VMFile

#set policy
Set-WBPolicy -Policy $WBPolicy

#start backup
Start-WBBackup -Policy $WBPolicy

I've remarked using FileSpec from the above script, which oddly enough actually works when enabled and commenting out the actual virtual machine addition to the policy.. which I thought was suppose to do the 'hot backup' or child vm snapshot.

Just a couple notes, the virtual disk is a differencing disk placed in a separate volume from the parenting vhd.  Is this a problem?  It doesn't seem to be one when I use FileSpec and grab the VM without the use of integration services.  I can actually watch the host build the temporary snapshot of the volume which retains the differencing VHD and when the backup is complete it is removed.

It is just trying to get integration services to make the hot backup causing problems.  Am I missing something from the policy or is there something obscure in the VM that needs to be checked?  I looked at the volumes within the virtual machine, and there is plenty of space available for VSS and the hosting volume does too...