ERROR SOLVED - [Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]

We are currently working on an ASP.NET website, which has lots of AJAX.

We have succesfully deployed it to our development server, running on Windows Server 2008 x64.

Our client has Windows Server 2003 32 bits, and as expected, the problems started right at the login page.

The website presented us with the error:

"Could not load type 'System.Web.UI.ScriptReferenceBase' from assembly 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"

At first I thought it had something to do with their server being x64, but then I found out it was x86, so no problem there.  Then asked them if the server had .NET Framework 3.5, it had too,  the only thing missing was .NET Framework 3.5 Service Pack 1.

So if you consider switching  your Script Manager or anything like that, don't do it, because it's literaly this simple: just install .NET Framework 3.5 Service Pack 1, and it works!

Hope this helps someone.