What is "Error code 4" when trying to restore a backup via Apple Configurator?

I am currently managing 60 iPads at a K-12 School. We use apple configurator to manage them and I am having an issue when I try to restore the iPads to a backup, a few of them come up with "error code 4" and do not restore the back up. I end up having to restore them individually. Is any one else having the same issue or know a way to fix it?