Does the error occur only when using our RadAjaxPanel or is generated when replacing it with plane MS UpdatePanel? Basically, if your logic is executed without issues when having regular update panel, it should operate as expected with our RadAjaxPanel (which is built on top of the MS UpdatePanel), too.
If the problem surfaces with RadAjaxPanel only, I will appreciate if you assemble a working version of your project and send it enclosed to a formal support ticket. I will familiarize with your code implementation and will do my best to provide more to-the-point answer/solution.
the Telerik team
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