After spending hours trying to get Fiddler to decrypt IOS HTTPS traffic (that in the past always have worked), I finally stumbled upon this article on how to get it to work
https://textslashplain.com/2016/07/27/using-fiddler-with-ios-10-and-android-7/

I think the official Fiddler documentation need to mention the need to manually go to Settings -> General -> About -> Certificate Trust Settings to make things work and to get rid of the (I guess) countless questions about how to solve this error in the log

SecureClientPipeDirect failed: System.Security.Authentication.AuthenticationException A call to SSPI failed....

A second improvement is to mention that to verify that the required Cert maker is installed correctly, you should see the BCCertMaker mentioned under Options -> HTTPS