Inaccessible virtual machines cannot be deleted occasionally in Virtualbox.

Fix it by issuing the below commands in a linux or Windows-DOS terminal.

To get a list of VMs run:
# vboxmanage list vms

The inaccessible ones will show up similar to:
"<inaccessible>" {52a91e62-096d-4f8b-96f4-5fa76db3cd76}

Now use the GUID in the unregister command:
# vboxmanage unregistervm {52a91e62-096d-4f8b-96f4-5fa76db3cd76}

Problem solved.

A related article to this problem can be found here: