Enable per node serialization. This is done by adjusting this registry key:
HKLM\Software\Microsoft\Microsoft Data Protection Manager\2.0\Configuration\MaxAllowedParallelBackups
Simply change the DWORD value Microsoft Hyper-V from the default of 3 to 1. To finish off restart DPM server and modify Protection Groups for virtual servers.
For more detailed description, read this technet article and this.