Is the OS on the VM updated with SP2 ?
What errors are logged in the VM's event viewer and in the Hyper-V host's VMMS logs ?
Are the VSS writers in a stable state ?
Lastly, try these steps as well. If step 1 works, then no need to follow step 2.
1. On the affected virtual machines;
a. Edit Settings of the Server from the Hyper-V console
b. Under Management > Integration Services uncheck the "Backup (volume checkpoint)" Select "Apply"
c. Now that its unselected, reselect the option and select "Apply"
d. Run the job
2. On the affected virtual machines;
a. Edit Settings of the Server from the Hyper-V console
b. Under Management > Integration Services uncheck the "Backup (volume checkpoint)" Select "Apply"
c. Run the job
d. Once it completes, reselect the option and select "Apply"
e. Run the job