I think the tablet UI is completely broken in the latest build of RootBox (20/3/2013), when selecting tablet UI in the AOSP app under the general UI tab, you will get a crash message "SystemUI has stopped working", which you will get all over again until you switch either to PhoneUI or PhabletUI.
More info : I had darkrootbox activated (I did tested also without darkrootbox activated but same result) and a custom theme (Crystal 2 for CM10.1/CM9) activated when I made the logcat
Logcat : http://pastebin.com/VQbwGrXx
Major edit : If you try to switch to tabletui without having a custom theme installed it does work.
Major edit 2 : seems a theme error, not a error in your ROM, the tablet UI does work with a few other custom themes. You may ignore this bug report.
Edited by thomcd, 22 March 2013 - 11:30 AM.