I saw someone else that a similar issue and it turned out to have nothing to do with the theme. (http://support.themeblvd.com/forums/topic/visual-editor-tab-shows-blank-page/) — to which I still do not know why this is happening to them, but none the less started for them after updating to WP 3.5 no matter what theme they have activated. Most likely if you’re having this issue and it were theme related, I think it would have been happening before you updated to WP 3.5, as well.
Have you tried activating Twenty Twelve temporarily to see if the problem goes away or not? You won’t lose any theme settings by doing this.
 Searching on wordpress.org forums, this seems to be a huge thing that many people are facing after updating to WP 3.5.