You are trying to backup vRanger VM with itself and you get a similar error to "One or more disks from vRanger virtual machine is already mounted to the vRanger virtual machine.
The problem is that vRanger 5.2.x is unable to back it self up no matter what kind of backup you are performing. And that is because vRanger 5.2.x does a mounted disk check for all types of backup job.
Upgrade to vRanger 5.3 to resolve this issue. vRanger 5.3 no longer does a mounted disk check unless perform a Lan-free backup is chosen in the backup job configuration. And allows the vRanger VM to be backed up by itself.