I was getting this error when clicking my AdvancedDatagrid. This only seemed to occur after converting the app to a module.

The answer to the problem was to add the following to your main application.
Hope this helps you spend a lot less time than I did resolving the problem

import mx.managers.IDragManager;
private var iDragManager:IDragManager;

Here is the exact error message:
TypeError: Error #1034: Type Coercion failed: cannot convert mx.managers::DragManagerImpl@2256be49 to mx.managers.IDragManager.
at mx.managers::DragManager$/get impl()[E:\dev\3.0.x\frameworks\projects\framework\src\mx\managers\DragManager.as:152]
at mx.managers::DragManager$/get isDragging()[E:\dev\3.0.x\frameworks\projects\framework\src\mx\managers\DragManager.as:187]
at mx.controls.listClasses::AdvancedListBase/dragScroll()[C:\Work\flex\dmv_automation\projects\datavisualisation\src\mx\controls\listClasses\AdvancedListBase.as:6617]
at Function/http://adobe.com/AS3/2006/builtin::apply()
at ()
at SetIntervalTimer/onTimer()
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()

Greg FLEX adobe, as3, exact error message, Flex 3 Errors, mx