Why not just use manual focus?
That's always an option, but my post is less an issue of manual vs auto focus as it is an issue of whether haze (or other particles in the air like smoke, fog, etc) will throw off auto focus. Besides, have you ever tried to manual focus with the 50 1.8? The focus ring is impossibly small and not very smooth.
Sure, but you're 250' away from the stage, something is making the AF focus closer than that (the fog, the crowd, the guy at the board). It seems like manually focusing at infinity would be the way to go, set it and forget it, unless you are also getting shots around the scene at different depths. Then I totally understand not wanting to manually focus in the dark with the tiny ring on the 50 1.8.
If nothing else was in the region of your AF points, I think you've kind of answered your own question, that the environmentals used at the concert are messing with the AF.