This is an interesting problem...
I've got an ASP.NET 3.5 web application. It's relatively simple - a content page based on a master page, with a couple of usercontrols on the page.
There are a total of four controls on the page; two are custom controls and two are Ektron CMS controls.
For some reason, I've been getting 'Sys is undefined' Javascript errors. The Firebug error console also throws an "ASP.NET client framework failed to load" error. I can't figure out why this is happening.
The kicker - if I take the controls that don't work completely out of the content page, they work just fine. It's obviously something in the content page trashing the framework, but I haven't been able to figure this one out.
I've looked through several dozen articles before posting, including suggestions like:
Has anyone seen something like this before? It's got to be something in that content page, but I'm struggling with what.
EDIT
I noticed while stepping through the code the UserControl's PAGE_LOAD event fires three times; on the third time the code throws an error. (I'm still trying to get my head around the error that's being thrown)
EDIT
I wanted to add this in case someone else sees this error. It turns out the problem was being caused by a Scriptmanager on the page template that wasn't needed.
When I took the Scriptmanager off the page, everything worked fine and the framework load error went away. I figure this problem must have broken other scripts and prevented them from running later on down the page.
If you get a 404 on ScriptResource.axd, there must be something wrong with your ASP.NET installation. I had this before, when I did not configure the correct ASP.NET pool. For instance: there could be another ASP.NET application on the same server assigned to the same pool, that is running under ASP.NET 2.0. You say your application runs under 3.5. Can you try iisreset on your machine and then start your application?
The pool could be assinged to the wrong .NET version. Another option could be, that .NET 3.5 is not installed on the server at all.
Are you sure that the site runs well without your content page? ScriptResource.axd is a dynamic handler to generate JavaScript on the fly. If it's not available, the scripts cannot work.