Alguma das VMs que você está tentando fazer backup contém discos dinâmicos? (Isso é no gerenciamento de disco dentro da VM, discos dinâmicos em oposição a discos básicos. Não expandindo dinamicamente VHDs)
When you perform a backup of the virtual machines, you must back up all volumes that host files for the virtual machine, including the InitialStore.xml file (in C:\ProgramData\Microsoft\Windows\Hyper-V, by default) and the volume(s) containing the VHD(s) and configuration XML files. For example, if the virtual machine configuration files are stored on the D: volume, and the virtual machine virtual hard disk (VHD) files are stored on the E: volume, and InitialStore.xml file is stored on the C: volume, you must back up the C:, D: and E: volumes. Virtual machines that do not have Integration Services installed will be put in a saved state while the VSS snapshot is created. Virtual machines that are running operating systems that do not support VSS, such as Microsoft Windows 2000 or Windows XP, will be put in a saved state while the VSS snapshot is created. Virtual machines that contain dynamic disks must be backed up offline.