The RadXmlHttpPanel uses ASP.NET client callbacks to retrieve its HTML from the server. The page does not go through its standard lifecycle - most importantly the ViewState is not loaded and saved. This means that, if you change a property of a server control, the new value will not be persisted. That's why the value of the TextBox is lost.
More information about the RadXmlHttpPanel control can be found on our website: http://www.telerik.com/help/aspnet-ajax/radxmlhttppanel.html
All the best,
the Telerik team
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