I have noticed that you are making a conditional post back to the server which is initiated by the control with ID="MenuBar". Make sure you have such control on the page.
Also having RadAjaxPanel inside an UpdatePanel is really not a good approach, since both of them are doing almost the same. For more complex scenarios, we advice to use a RadAjaxManager, about which information could be found here: RadAjaxManager
For more information about using a RadAjax in UserControls scenarios, you could refer to the following help article on our website: RadAjax and WebUserControls
Hope this helps.
the Telerik team
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