For the past several weeks (well before the Midnight prepatch addon changes) one of my warlock toons has been unable to cast Demonic Gateway. The combat log reports “Your Demonic Gateway failed. (You can’t do that right now.)
I have tried unlearning and re-learning the talent, changing specs, tried casting it from a macro, tried casting it in dungeons (I used to use it to skip a pack of mobs in Utgarde Pinnacle) and outdoors, in cities, everywhere I could think. I receive the same “You can’t do that” message.
I have other warlocks that can cast the ability. Not having this utility on my main is really bothersome. I’ve seen several topics on here with people reporting an issue but no posts on how to resolve or if a fix was found.