The problem is most probably related to the fact that your application is running on the 4.0 version of the .Net Framework. Unfortunately RadAjaxManager does not work in Master/Content pages scenarios with the publicly available version of the framework due to a bug in it. You can read more details about this issue in this forum thread
. To workaround it I can suggest to set the ClientIDMode of the page to "AutoID".
I can also inform you that the issue is fixed by Microsoft and everything should work for the final release of VS2010 and .Net 4.0.
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