Please check http://kb.vmware.com/kb/1010310.
Also check your backup server if it has a file from the VM still attached.
If the backup server is a virtual machine, check with "Edit Settings". If there is one attached, remove it (NOT delete from disk, just removing it).
If it is a physical server, reboot it to make sure all locks are released and orphaned processes are stopped.
If the VM is powered off (you said you killed it), find out if there are still running processes for the VM or locked files.
The below steps only make sense if the VM is powered off (otherwise of course processes are running and files are locked).
1. Go into home folder of virtual machine and run "touch *". This will check for any file locks.
So you would get "Device or resource busy" if files are still accessed for this VM.
2. esxcli vm process list
-> Shows you if the VM is still running
3. ps | grep <VMname>
4. vmkvsitools lsof | grep Virtual_Machine_Name
Will show you if there are still worlds/cartels running accessing the VMs disks.