Please note that the "RadAjaxManager" control for ASP.NET AJAX has no "OnRequestError" event. This event was present in our oldest (no longer supported) controls for ASP.NET (you could read about the migration from RadAjax for ASP.NET to RadAjax for ASP.NET AJAX here
Nevertheless, I would highly recommend that you upgrade to our latest RadControls version, since lots of improvements were introduced to our product.
As for the error handling you could have a look at our help article "Error Handling for Ajax Requests
Hope that helps.
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