The Custom Sidebars plugin by WPMU DEV was recently updated from v2.1.0.2 to v2.1.0.8 and the Widgets screen is now fairly broken. The sidebars no longer have a "Sidebar Location" button, nor a "Allow this sidebar to be replaced" checkbox. Additionally, You cannot scroll the sidebar area of the "Widgets" page. Good thing I made a backup before I upgraded it. I restored that backup and things are at least back to where they where. I'm not sure who's responsible for this issue, but there is definitely some sort of incompatibility with the new release and the UUA theme.