Ajax - 'sys' in undefined and the ASP.NET Ajax client-side framework failed to load

I had been struggling with this issue for a couple of days now. A small warning icon appears in Internet Exporer. Clicking on it shows a number of error messages.

1 - ASP.NET Ajax client-side framework failed to load
2 - 'sys' is undefined

I had checked the web.config file multiple times, all required sections were present and referencing the correct .net version dlls.

Finally after double checking absolutely everything I noticed that one of the handlers the web config was referencing did not have an extension mapping. This was the .axd extension. I added this in to IIS but the problem still occurred. Checking the handler mapping again I unchecked the 'Verify that the file exists' check box and retried the web page. This time it worked!

So in conclusion it turns out you do need to have the .axd extension mapping if it does not exist. And the check box to 'Verify the File Exists' must not be checked.