When backing up VMs through a VCenter server controlling fully licensed ESXi 5.x hosts, the backups can fail if the backup user ID is not correctly configured.
Backups through VCenter will fail with the following error:
Error 2014/09/17 15:23:19 152 Data Plugin SERVER TaskID 1 Failed to open virtual disk
With context
The VCenter Server level user ID needs to be assigned the following permissions to run backups:
Global > DisableMethods and EnableMethods
Global > License
Further information can be found on the following Knowledge Base article from VMware:
http://kb.vmware.com/kb/2063054