When using Telerik, you may get the following Javascript error:

ASP.NET Ajax client-side framework failed to load

If you check with Firebug, it shows that the browser gets a 404 error for the file Telerik.Web.UI.WebResource.axd.

In my case, I had the following in my Web.Config:

<add name="Telerik_Web_UI_WebResource_axd" path="Telerik.Web.UI.WebResource.axd" type="Telerik.Web.UI.WebResource" verb="*" preCondition="integratedMode,runtimeVersionv2.0"/>

I removed the ,runtimeVersionv2.0 part and it worked.