When this problem occurs it is a permissions error on the slave node.
To repair this problem login via SSH to the slave node. Go to the directory /vz and check the permissions of two folders, the root folder and the private folder. Both will likely be set to 700, when they should be set to 755. Change the permissions to 755.
In order ensure your virtual machine starts, you will now need to reinstall the operating system for the container.
