Can you please check whether the second error appear if you set EnableAJAX = false for your RadAjaxPanel (wrapping the ReportViewer control) or replace it with standard MS UpdatePanel instance? If the problem persists, I suppose that the ReportViewer is somehow not compatible with the ASP.NET AJAX framework and you will need to find a method to bypass the limitation searching in the ASP.NET AJAX forums.
Once you make the configuration fully operational, re-enable the ajax for the RadAjaxPanel and you should be done.
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