Please remove manually the RadControls for ASP.NET AJAX from the toolbox of Visual Studio and use the approach suggested in this help article
in order to add them again by referencing the latest version of Telerik.Web.UI.dll.
I hope this helps. Feel free to contact us again if you encounter more difficulties.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed