Backups for one or more vm may fail with this error message. A similar message will be logged in Virtual Center for the affected vm.
Attempting to create a snapshot from Virtual Center results in the same error.
This error message is being given to vRanger by the VMware environment in response to vRanger's API call to create a snapshot.
If it is happening to only 1 vm attempt to vmotion the affected vm to another ESX(i) host.
If it is happening to multiple vms on the same host a restart of the VMware managemt services on the host and Virtual Center may be required. For more information on this proceedure please refer to VMware KB 1027707http://kb.vmware.com/kb/1027707