Based on the provided information it is hard to say what is causing the described issue on your end. Could you please post your aspx page declaration with the related code behind? Also could you please disable the ajax from the page (by setting EnableAJAX="false"
to the RadAjaxPanel) in order to see the real exception message when the error is thrown.
Looking forward for your reply.
All the best,
the Telerik team
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