Upfront editor will not open

OK, so I needed a separate page for sign-in and registration so I downloaded a couple of plug-ins to do this, installed them on separate pages and Upfront Editor now refuses to load. I don't *think* its these plug-ins as I disabled them and the editor still wont start.

I've tried from in the dashboard: Appearance/Themes/Customise and from the site itself.. the editor goes to the "loading..." page and never comes back. I've waited over an hour in some cases.