The VM has "stuck" open snapshots, and they cannot be consolidated because the zero free-space will not allow the snapshot removal process to run. VMware displays an error to this effect saying there is not enough room to add/delete snapshots (visible in the VMWare client).
If a snapshot removal procedure call from vRanger to the Virtual Center or ESX(i) host fails for some reason (power outage, nested "orphaned" snapshots on the VM, etc.), the snapshot may not be closed successfully, and will continue to grow on the VM. This could fill-up the datastore LUN on the ESX(i) host that hosts the VM in question. If disk space one the VMWare datastore LUN is close to zero, no new snapshots/snapshot removal procedures are possible, due to having no room to execute the consolidation procedure.
Free-up disk space on the datastore LUN in question temporarily, so that the process can run to consolidate the snapshots. Here are some suggested ways to do this: