Hello Ade,
The descried error appears in cases you have nested UpdatePanels on the page. This means that if some control is updated with RadAjaxManager and with RadAjaxPanel or asp UpdatePanel at the same time the error could appear. Please elaborate a bit more on your scenario and the Ajax settings you have so we could further research on the problem.
All the best,
Maria Ilieva
the Telerik team
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