I receive the following error after trying to merge a working webpage to another website.  The ajaxcontroltoolkit.dll is referenced properly in the new site.
 

 Unable to cast object of type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection' to type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection'.
Source:  System.Web.Extensions
-> System.Web.Configuration.ScriptingScriptResourceHandlerSection.ApplicationSettings.EnsureSectionLoaded()
-> System.Web.Handlers.ScriptResourceHandler.IsCompressionEnabled(HttpContext context)
-> System.Web.Handlers.ScriptResourceHandler.RuntimeScriptResourceHandler.System.Web.Handlers.IScriptResourceHandler.GetScriptResourceUrl(Assembly assembly, String resourceName, CultureInfo culture, Boolean zip, Boolean notifyScriptLoaded)
-> System.Web.UI.ScriptReference.GetUrlFromName(ScriptManager scriptManager, IControl scriptManagerControl, Boolean zip)
-> System.Web.UI.ScriptReference.GetUrl(ScriptManager scriptManager, IControl scriptManagerControl, Boolean zip)
-> System.Web.UI.ScriptManager.RegisterScripts()
-> System.Web.UI.ScriptManager.OnPagePreRenderComplete(Object sender, EventArgs e)
-> System.Web.UI.Page.OnPreRenderComplete(EventArgs e)
-> System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Open you web.config and see if the following line appears.  If not, your web.config is probably not set up properly for AJAX. 

<section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>

Yes I have that line.  The only difference is it has version of 3.5.0.0.

 
The main difference between the old website that works and the new one is I was using the asp.net futures july release on the old one.

I found a solution:

http://www.telerik.com/community/forums/thread/b311D-hhmah.aspx#429929