We are aware of this problem on our online demo. It is a result of an Ajax conflict caused by quickly dragging one or more docks. When a dock is moved an ajax request is initiated to update the dock state on the server. If the dock is dragged very quickly multiple ajax requests are initiated and this causes a problem.
To fix the issue in your application an AjaxLoadingPanel should be displayed while the ajax call is executing, to make sure the user does not initiate a second request, as shown in the following Code Library article
We will update the demo as soon as we can.
the Telerik team
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