The exact same thing has been happening to me, as well (alt-tabbing back into the game and having it crash %100 of the time). I've even tried a fresh install of FFXIV to be sure. I'd never had this issue occur during beta and such. In fact, I was ecstatic that I even could, compared to other MMOs/full-screen games. If possible, please address this ASAP, SE, so that the feature doesn't seem too good to be true!