I had the same glitch a couple days ago. I restarted the xbox and it seemed to do the same thing, but then just as I was about to give up the save game popped up and then it was fine. Sorry, that prob doesn't help much.
Is there a way to change team colors that I missed? I've been playing co-op with my son and the "dark blue" vs "light blue" on our team can get very confusing. The enemies are all red, so why are we stuck with just two slightly different blues? Green? Orange? Purple? Just give me a couple choices. Also, it would be really nice if the little character icons at the bottom would highlight in some way when I had that character selected. The game is really fun so far though, so that's nice.