I was not able to access any websites when Fiddler was open when using Microsoft Edge.
Fiddler enters http=127.0.0.1:8888;https=127.0.0.1:8888 into MS Edge’s Manual Proxy setup.
What I did to workaround the issue: Removed the :8888 for each http/https address and then entered 8888 for the port number.
I can confirm this bug in Windows 10 version 15048; it's a bug in the "Modern" Control Panel's Proxy settings, whereby it doesn't properly parse a WinINET proxy configuration string. Microsoft needs to fix this but it should be harmless if you don't open the Modern Control panel while Fiddler is running.
Fiddler doesn't do that, but it looks like you've found a bug in Microsoft Edge or in Windows 10.
What build of Windows 10 did you encounter this behavior on?