Myself and others have the same problem. The problem does not exist in 4.0. I have it in 5.1, and your post helps to define when the problem started. Beware when using GhettoVCB to back up. First says the backup happened fine, which it didn't, then says there is an error with one of the virtual disks, but doesn't tell you what the error is, and then it deletes your good backups, and replaces them with the broken backups, so in a few days, you won't even have old backup to go restore from.
The way I work around it is to make the backup volume its own separate VHD, then when it has a problem, I make a new VHD. This is a pretty obvious problem, and apparently has been going on for quite some time if you have it also with 5.0. Thus, I don't know if it's introduction was intentional to defeat using vkmfstools for backing up or not. There is certainly an incentive for doing so. I'm glad I didn't upgrade my VMs so that I can go return to 4.0.