Reading around particularly on Apple's support and discussions site apparently I'm not the only one with this issue. Some others are having the same problem connecting to their WPA-2 personal. It might be something Apple has to fix.
I was going to mention those support forum posts, but I figured you were already reading them. I've seen many WiFi problems on newer equipment in the last few years—always during an upgrade. Perhaps Apple is using a different manufacturer's WiFi chips, or even a mixture of equipment from different manufacturers.
Again assuming you've considered this sort of thing, try all the variables on the router: 2.4 vs 5 GHz, different security vs none, etc.