I have also started experiencing this on one of my sites but it is occuring in IE 7 (though it does not occur in IE8) as well as Firefox
I am running 2.2.3. I get a 500 Internal Server error via Firebug.
The problem only occurs on the menus with 3+ options. If I trigger the problem, click out of the Site Content tab and then back in the menu will by open when I go back in.