V-79-40960-38467 - Virtual machine SERVERNAME has been excluded from the backup because it must be put in a saved state to back up

There have been several other posts with this error. Most of them unresolved. I have tried everything in them, hence new post.

Hyper-V Host running Windows Server 2012 R2 Datacentre and Backup Exec 15

All updates for Backup Exec installed (same for Windows)

2 Virtual Servers included as Hyper-V backups both report:

V-79-40960-38467 - Virtual machine "Servername" has been excluded from the backup because it must be put in a saved state to back up.

There are no other servers (virtual or physical), just the Hyper-V host and 2 guests. One Guest runs Windows Server 2012 R2 Datacentre. The other runs Windows Server 2008 R2 Standard.

I have run through the following articles:

https://www-secure.symantec.com/connect/forums/v-79-40960-38467-virtual-machine-servername-has-been-excluded-backup-because-it-must-be-put-s
https://support.symantec.com/en_US/article.TECH64287.html
https://support.symantec.com/en_US/article.TECH209419.html
https://www-secure.symantec.com/connect/forums/backup-exec-puting-hyper-v-vm-saved-state

Although for previous versions of Backup Exec and Windows Server, ran through posts anyway. Everything checked already.

I cannot use the option to untick the "Exclude Virtual Machines that must be put in a saved state to back up" as the servers have software that should be available 24/7 ideally. Also, you really shouldn't have to do this.

SCVMM issues (2008 R1).
We have never had SCVMM installed. Purely Hyper-V management
The Hyper-V integration services are installed and the VSS integration service is enabled (inc. COM+, DTC services as well)
Yes. Both virtual servers have latest integration services and have all the services enabled
All virtual machine disks are configured within the guest operating system as NTFS-formatted basic disks.
Yes
Each virtual machine volume must specify itself as the storage location for its shadow copies
Yes
All integration services enabled on Hyper-V settings
Yes - all other than Time synchronisation on one of the servers due to it being the PDC-E Domain Controller and syncing from external host
Second server (not a DC) has all Integration Services ticked
Ensure 20% disk space free on host and VMs
Yes
Ensure plenty of un-fragmented RAM is available on the host.
Yes
Backup Exec agents installed on VMs
Yes
Add reg key: HKLM\Software\Microsoft\Windows NT\CurrentVersion\SystemRestore --- REG_DWORD:ScopeSnapshots:0x0
I have not done this yet. Also article says this is for Windows Server 2012 guest only. One guest is 2012 R2, other is 2008 R2. Reluctant to add change unless I know exactly what it does and why - no explanation provided
Run backup as Remote Agent instead of Hyper-V VM selection
I have not done this yet and will try at earliest opportunity but not really a good solution