I suppose that the issue you are facing appears in case the user clicks several times to select different items before the previous request has finished.
By design ASP.NET AJAX Framework cancels the ongoing ajax request if you try to initiate another one prior to receiving the response from the first one. By setting the RequestQueueSize
property to a value greater than zero, you are enabling the queueing mechanism of RadAjax, that will allow you to complete the ongoing request and then initiate the pending requests in the control queue.
Give this a try and let me know if it works.
the Telerik team
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