OK, it sounds to me like the virtual machine still remembers the last location it launched from is looking to launch it from there now, but the file/Owner doesn't exist anymore.
First, in the Virtualbox main window, right-click the virtual machine called and click "Remove, then "Remove Only". The virtual machine will disappear out of the list. Close the Virtualbox main window.
Then, find virtual machine folder (the one with .vbox in it).
Double-click the .vbox file. A new Virtualbox main window should open in a few seconds, with the virtual machine in the list.
You should now be able to run it.
