In order to overcome this issue I suggest that you turn the RestoreOriginalDelegate property back to true. And in order RadAjax to work under medium trust, try inheriting your page from RadAjaxPage instead of Page.
If you still prefer setting the RestoreOriginalDelegate property to false, then I suggest that you register the scripts through the ScriptManager.RegisterStartupScript() method instead of using the ResponseScripts property of RadAjax.
Check it out and let me know if it works for you.
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