When you perform some action (transfer/reorder) with the RadListBoxes
the changes are written to the client states of the both listboxes. This is done in a purpose to save the state to the server and updating the viewstate of the both controls. When the actions with the RadListBoxes take place, they both clear their client states. If you don't manage to update the state of the controls after performing the actions the client state is not correctly cleaned. Here is the root of the problem and the error you have faced. The viewstate of the page is updated by RadAjaxPanel
and the changes made to the listboxes could be saved there. Therefor i recommend you to use RadAjaxPanel
in order to correctly update the client state of the both controls and escape the experienced issue.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed