@olivierlambert Yes I get the same result on stable vs latest channel.
@florent The ones that fail do seem to be some of my larger VMs the Windows 10 VM that I have been testing with is about 88.3 GB used according to the OS.
All of my VMs are thin provisioned and our NetApp storage is using de-duplication so the size of the VHD on my storage is significantly less in this case it was about 3 GBs.
As a test I created a new storage volume without thin provisioning and de-duplication. I migrated the Disk to the new volume and the VHD is 103 GB. I also removed de-duplication and compression on my remote. I tried the backup again and it failed with the same error.
I do see some errors from the xensource.log around the time it fails and I attached those here.
I am editing my post after looking at the log file @Delgado posted mine are slightly different. I added mine for comparison. At some point during my testing the error also changed slightly and started stating VDI must be free or attached to exactly one VM. It appears after a failed backup it's not cleaning up the snapshots.