I've had this problem before. For me it was an addon that was 'hidden' in the middle of my screen so I couldn't see it but I also couldn't use the space to move my camera. I can't remember exactly what addon it was but it could have been a hud of some sort. I almost want to say that my hidden Bartender4 bars at one point were causing this as well until I moved my 'hidden' bars somewhere else where I didn't click. This happened to me quite awhile ago...sorry I don't have more specifics.