I discovered a conflict today that EMAIL USERS disables the ability to show, hide and therefore select Pages in the Appearance -> Menus section of the WordPress dashboard.
I'm using the latest theme from Elegant Themes that supports one custom menu. With Email Users activated I'm unable to show and hid the Pages pane on this page. Also unable to use any of the show / hide functionality for Categories and Posts.
here is a quick screen cast that shows the conflict: