When you upgrade your site to use the ASP.NET AJAX Beta 2, you must add a new HttpHandler in the web.config file. Under the <httpHandler> element in the web.config, add the following handler:
<add verb="GET"
path="ScriptResource.axd"
type="Microsoft.Web.Handlers.ScriptResourceHandler"
validate="false"/>
Otherwise you may receive 'Sys' is undefined error message