Virtualbox itself relies on Qt, so I'm wondering if this isn't caused by a particular version of Qt being shipped with the Windows installer of Virtualbox. I'm assuming the OP's host machine is Windows as well? The discussions on the virtualbox forums mentioned Linux hosts, which would probably rely on the system Qt. So an update to the system Qt could have made the bug disappear if something changed on the Qt side of things (not that anything like that would ever happen
).