Actually, I didn't get a 404 with UWS Pro. I got a directory listing. Attached project has screenshots running in ASP.NET WebDev, UWS Pro 32-bit, UWS Pro 64-bit, and IIS Express 7.5.
WebDev and IIS Express worked as expected.
UWS Pro, regardless of bitness, shows a directory listing.
Note that the issue seems to only exist with the default route "/". If I add another route:
Get["/hi"] = _ => "Hello World 2";
UWS Pro responds to "/hi" as expected.