It is not possible to create a backup of a Windows virtual nachine with dynamic disks:
# prlctl backup myvm Backing up the VM myvm Failed to backed up the VM: The backup command for the "myvm" virtual machine failed with code 25 ("Backup error"). Try to back up the virtual machine again. If the problem persists, contact the Parallels support team for assistance.
The issue is known as a product bug with ID PSBM-10325.
The bug was fixed in Virtuozzo 6 update 8. Install pending updates in order to resolve the issue.