Error: Expected identifier while developing simple website in AJAX 1.0 RC

It sounds very strange that the exception?'Sys is undefined'?takes?place?at?the?second?time?to?run?the?website in the browser.Try to consider the following two suggestions.
1.Try to take a look at the following explanations about Known Issues in the RC release
----------

* ASP.NET AJAX applications running with IIS 7 on Windows Vista do not run in application pools configured in
"Classic Mode". Calls to the application return a "500 - Internal server error". You can work around this
problem by running in "Integrated Mode".

* The WebResourceCompressionModule HTTP module has been removed in the RC release. ASP.NET AJAX scripts are now
loaded using the ScriptResourceHandler.axd HTTP handler, and WebResourceCompressionModule is no longer needed.
If your applications used a previous release of ASP.NET AJAX, you must remove the following section
from the Web.config file to avoid a compiler error when building the application. In the <httpModules>
configuration section, you'll see the following, or similar configuration:

<httpModules>
<add name="WebResourceCompression" type="System.Web.Handlers.WebResourceCompressionModule,
System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
</httpModules
You must then remove the following line:

<add name="WebResourceCompression" type="System.Web.Handlers.WebResourceCompressionModule,
System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/
The resulting section will appear as follows:

<httpModules>
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0,
Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
</httpModules
2.To avoid this problem in ASP.NET AJAX, be sure not to attempt to use any of its client side libraries until it has raised the pageLoaded event,
which signals that it is fully initialized and ready.
Here's more info:
http://ajax.asp.net/docs/ClientReference/Sys.WebForms/PageRequestManagerClass/PageRequestManagerPageLoadedEvent.aspx?

Wish the above can help you.

hello.

have you published your app? if yes, can you send us a link to test it?