This keeps happening to me many times. I don't really care about the achievments, I just want to beat it. There is a code to unlock all the missions, so you can start playing from where you lost your game saves. At the mission select menu hold both the left and right bumpers, and quickly put d-pad left, left, right, right, Y, Y. If done correctly, a horse will sound. Sorry is this is a repost.

Yes this is happening to tons of people. I've emailed activision support about it many times and they said they were currently researching the issue and hoping to release a patch soon so we'll see.