The most common
reason this happens (particularly when it occurs in multiple browsers, and just after you upgraded) is that you have a 3rd party firewall product installed and that firewall is blocking all connections to Fiddler. You'll need to configure your firewall to allow access to Fiddler.
reason this happens is if you have a Windows Phone device plugged into your PC via USB. The Windows Phone team chose port 8888 as their debug port, which prevents traffic from going to Fiddler when the phone is attached. You can resolve this problem by configuring Fiddler to run on a different port.
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