I’m afraid this isn’t quite what I thought and looks like is going to be a more challenging issue to tackle that I unfortunately don’t have the answer for you on.
Your issue doesn’t have anything to do with the theme or any plugins you’re using. I activated the default Twenty Twelve theme and de-activated the one plugin you had which was All-in-one-SEO, and you still had the exact same issue when editing pages and posts. So, this suggests something is off with your WP installation.
I’ve personally never seen this before. You may want to try posting on WordPress.org forums and see if someone else has some more ideas as this is now just a general WordPress issue.