Trouble Adding Toolkit Controls

When I attempt to add a toolkit control, my site crashes. I have non-toolkit ajax controls working just fine, but as soon as I add an extender(always visible) my site crashes.

If I preview it on my developing machine, it works fine, but then I get the following on the production machine:

Unable to cast object of type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection' to type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection'.

Description:An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details:System.InvalidCastException: Unable to cast object of type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection' to type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection'.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidCastException: Unable to cast object of type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection' to type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection'.] System.Web.Configuration.ApplicationSettings.EnsureSectionLoaded() +70 System.Web.Handlers.ScriptResourceHandler.IsCompressionEnabled(HttpContext context) +7 System.Web.Handlers.RuntimeScriptResourceHandler.System.Web.Handlers.IScriptResourceHandler.GetScriptResourceUrl(Assembly assembly, String resourceName, CultureInfo culture, Boolean zip, Boolean notifyScriptLoaded) +30 System.Web.UI.ScriptReference.GetUrlFromName(ScriptManager scriptManager, IControl scriptManagerControl, Boolean zip) +293 System.Web.UI.ScriptReference.GetUrl(ScriptManager scriptManager, IControl scriptManagerControl, Boolean zip) +237 System.Web.UI.ScriptManager.RegisterScripts() +507 System.Web.UI.ScriptManager.OnPagePreRenderComplete(Object sender, EventArgs e) +111 System.EventHandler.Invoke(Object sender, EventArgs e) +0 System.Web.UI.Page.OnPreRenderComplete(EventArgs e) +2052172 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2247

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Version Information: Microsoft .NET Framework Version:2.0.50727.1378; ASP.NET Version:2.0.50727.1378

As always, thanks again for all of your help

I'm thinking it has something to do with IIS. If I preview it though VS2008's development server, it works fine, but when I run it through IIS is when my heartaches begin...