I haven't used Fiddler for a month or so. Now when I load it I can't connect to anything. It breaks all internet traffic. 407 error messages for all URLs including those that are from the actual server I'm running Fiddler on. When using a web browser I get a proxy server user authentication prompt window. It makes no difference when I put my credentials in and click submit. It just throws up the same credentials prompt over and over. If I turn on 'Rules\Require Proxy Authentication' and then turn it off again, (lots of times) eventually it starts to allow IE to start connecting. But if I put the url I'm testing into Fiddler itself in the Composer tab and execute a get request there, I continue to get 407 errors in Fiddler, even though that same URL is giving a 200 now in IE on the same box.
I have tested this under two different user accounts on two PCs, and the server admin account on two different servers. Results are consistent. Always a 407 error. Turning 'Rules\Require Proxy Authentication' on and off a number of times eventually allows IE to get access, but never allows Fiddler to send requests from inside the app in the composer tab. This is using the latest version of Fiddler.