Same thing happened to me...got in right away with a red spot and it wouldn't let me pay. I knew it wasn't something I did because I bought football parking at 8:45am. Eventually the number of red spots disappeared even though I had one in the cart. To be safe, I grabbed a green spot when there was only 5 of those left. But then went back in about an hour later and got a red spot.
So, if anyone needs a green parking spot I have one for sale!