What I would advise is that you first examine the following help article on the matter: http://www.telerik.com/help/aspnet-ajax/window-troubleshooting-dialogs-from-the-server-methods.html
for a simple alert with it you should get a similar error.
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