Recently I ran into a problem where browsing to a newly installed web app produced a bunch of javascript errors.  Stuff like: “‘Type’ is not defined” and “‘Sys’ is not defined”.

After debugging it for a while, I found the problem to be that URLScan had been installed on the server (Windows 2000 Server), which was preventing any requests with dots in the folder name.

URL Scan is a tool that MS suggested everyone install a while back that acts to filter out many malicious attacks.

So, with the default settings any request for a file inside the scriptsSystem.Web.Extensions folder would be denied as a 404 b/c of URL Scan.

To fix this, you need to edit the UrlScan.ini file, located in %WINDIR%System32InetsrvURLscan.  Near the top of the file, change AllowDotInPath from 0 to 1.

The run iisreset to restart IIS and you should be ok!

More info on URLScan is available here:

http://support.microsoft.com/kb/326444