I am having the same issues with Fiddler. I am also on Windows 10 Version 1703 and Fiddler version 4.6.20171.14978.
Everytime I attempt to capture traffic, I receive an error "Failed to register Fiddler as the system proxy.
Tried to change browser that Fiddler is using but don't seem to find how to do this. Fiddler appears to be using I.E. browser connection which is not working within Fiddler. However, if I just run I.E. within Windows, it works fine. Possibly this is the cause of the above error.
Can someone help?