Déjà Vu X starts in demo mode, or dongle does not work

If you copy of Déjà Vu X is activated by a dongle but that dongle does not work anymore, you may get a message saying that Déjà Vu is running in demo mode, or as the Editor edition, when you try to start it. Usually, the most straight-forward solution to this is to re-install the dongle driver.