Comment # 1 on bug 926243 from Wolfgang Bauer

Uninstall vboxgtk and install virtualbox-qt instead, that's the official GUI
included in VirtualBox.

vboxgtk never fully supported VirtualBox versions 4.2 and higher, and it
doesn't work at all any more. It seems to be dead upstream (last change was in
2012), so it's unlikely it will be fixed/improved any more either.
Therefore it has been removed from the distribution completely in November
already, but unfortunately too late for 13.2.

*** This bug has been marked as a duplicate of bug 902610 ***