This is not a WPMU theme or plugin issue, but thought you might see something. I have been using the theme Ultimatum. I got a blank screen in both admin and front-end right after the most recent Ultimatum update. I changed the plugins folder name to deactivate, but nothing came back. I changed the theme folder name for Ultimatum and was then able to get into the admin. Changed it back and reactivated it and back to blank screen. I am currently running 2014. I have enabled support access. I have tried to contact the developer, but have had no luck.