Hello Fred,
Our system indicated duplicate support ticket and forum thread on that issue. In order to avoid duplicate posts, please ask any further questions in the formal support ticket that you opened.
Additionally, I am attaching here a sample project handling the described scenario without any errors being thrown (having RadAjax and AjaxControlToolkit controls in one and the same page under .NET 4.0) for others to see.
Greetings,
Tsvetina
the Telerik team