Hi!
Your application under Cassini can have any number of subfolders as long as those subfolders are physically located under the root folder of your applications. For example, if your Cassini web application is installed in "C:\MyApp\" folder, then a script located at "C:\MyApp\Scripts\JS\someScript.js" will have a an absolute URL of /Scripts/JS/someScript.js for all of the application pages. Relative URLs will works just as fine. However, Cassini won't be able to get access to scripts located in, say, "C:\GlobalScripts\" folder, becasue it's not a subfolder of "C:\MyApp\". That is how it is in principal.

If information above doesn't help, then please try to find out what is the exact script URL that browser is going for when requesting the Infragistic scripts. You can do that by running a wonderful free HTTP tracer called Fiddler. If you have trouble reconciling the script URL with script's actual location, please contact us by email and let us know where relative to your application root your scripts are located, and what is the actual script URL requested by the browser.

Thank you for using UltiDev Cassini Web Server!
Best regards,
UltiDev Team.