VMware VCB works by creating snapshots of VMs in quiescing state.

If, while attempting to backup a VM, the following error shows up:

..  'vcbMounter' 3076465472 error] Error: Backup snapshot already exists.

..  'vcbMounter' 3076465472 error] An error occurred, cleaning up...

There are probably some hanging snapshots of the VM that need to go.
It happens often while using NFS in hard mount mode for offloading the compressed backups: if losing the share the VCB’s process simply hangs in “D” state (uninterruptible sleep) and the temporary snapshot pollutes your datastore.
The solution is simply to get rid of the snapshots: either by using the VC client or by performing the vmware-cmd removesnapshots, as for the following example:

vmware-cmd /vmfs/volumes/local_store_1/myVM/myVM.vmx removesnapshots

Keep in mind that the above command destroys all snapshots of the VM, you might want to recreate one.