As you probably know, Data Protection Manager 2012 R2 uses virtual machine snapshots to take a backup of a protected VM.
Unfortunately, when things go wrong with the DPM server, the backup snapshots might be left behind and you cannot delete it using the GUI.
In this case, you can remove this snapshot by using Powershell:
Get-VMSnapshot -VMName <NameOfYourVM> -ComputerName <NameOfTheHyperVHost> | Remove-VMSnapshot
Note that the above command will remove all the associated snapshots with the named VM, so be careful that you don’t have any you want to keep.