If you want to backup your Virtual Machines running on your Hyper-V server using the Windows Server Backup feature, you need to manually create the following registry key on your Hyper-V server:
Name: Application Identifier
First, if you not already done so, add the Window Server Backup feature from the Server Manager because it is not installed by default. Create the key and the String Value. You can also download this file, rename the extension from .doc to .reg and run it on your Hyper-V server.
Source: Microsoft KB 958662