Note that the problem is not with RadAjax controls specifically. You can reproduce it with an UpdatePanel as well. You can even see in this post
that the author of the engine claims that it does not support Web Forms:
We, as a rule, do not support scenarios which are unsupported by the MS AJAX framework, so if you encounter further issues with the engine not working with MS AJAX (and RadAjax which is built on top of MS AJAX), we cannot help you with them.
the Telerik team
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