Do you have an umlaut or a special character in your VM name? You must also look at the name of the disk-file, if a special character is masked or not.
If it is masked, it looks like this: +w7w-. If it is not masked you see the special character in the name of your vmdk file and since VDP V5.8 this is a problem.
Previous versions of VDP havn't had a problem with this. If this is the case you can solve it by renaming the VM and doing a Storage Vmotion to another location.
If you have set the parameter config.provisioning.relocate.enableRename to true, the disk-file will get the new name and the backup should work than.
This has solved the problem in my environment for 2 VMs. But I have also a few other VMs, where an manual consolitation is necessary, but not on a daily basis.
So there a more reasons for this problem, but I only have a solution for the umlaut or special character reason.
Paul