If your website use MSAJAX (System.Web.Extensions) , you may forgot to register it to web.config :
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
(for old version , it will be Version=1.0.0.0)
Tegards,
Terry