There's an error in UI/Droppables. I'm using Firefox 2.0.0.7 on Windows XP Home SP2. When I try to drag draggable element I get an error: oDrag.helperSize has no properties in ui.droppable.js (line 169)
The same error occurs on IE7.
I got the same error when I remove droppable/draggable li element from ul list, and recreate it in other list with the same id and properties. In attachment screenshot form Firebug with error that always occurs immediately before error in ui.droppable.js.