I am currently doing so research and development that involves working with the MS ASP.NET AJAX 1.0 Extensions and the Ajax Control Toolkit. Anyhow, after I created a couple of pages, I wanted to see how "heavy" they were on the client side. Basically, checking to see how much content (html markup, js files, css files, etc.) is being sent down to the client browser. So I fired up Fiddler2 to begin analyzing and was met with the following error in my browser:
[Fiddler] Connection to localhost failed.
Exception Text: No connection could be made because the target machine actively refused it.
So I jumped out to the Fiddler discussion forums and found the following post : New 2.0 Build - localhost refused. It turns out that Fiddler will try to use IPv6 if it detects it and my local IIS is not bound to IPv6.
The option to turn off the use of IPv6 is available from Tools->Fiddler Options on the General tab. As soon as I turned this off, everything was working again as expected. So if you are using Fiddler2 and run into this issue, turn off IPv6.