This has happened few times with one of our Vmware Data Recovery appliances running on VSphere. I have shut down the appliance, as it became unresponsive while still performing a backup of some VM’s into a Windows share.
After forcing the shutdown, the appliance wouldn’t power on again, I kept getting the message “Unable to access file <unspecified filename> since it is locked by System“. I searched long and hard, people were talking about rebooting the ESX host.
I found a simpler solution, if you have forced the shutdown of the vDR appliance, some of the VM’s harddisks would still be attached to the appliance and this could be checked and confirmed by editing the setting on the vDR within vSphere client. Normally vDR has only one harddisk attached to it unless you have mounted more harddisks for backup purposes. By removing these extra harddisks from the vDR, the appliance would power on again.
That same error would occur if you are trying to delete a snapshot attached to a VM while vmdk file(s) of that VM is mounted to a vDR. Removing the mounted volumes from vDR would release the handle on those disks and enable you to delete the snapshot successfully.