正解
The solution for the Fiddler error of "Failed to register Fiddler as the system proxy" is in the following link, but you have to reboot after applying it to make is active.
http://www.rudyhuyn.com/blog/2018/01/10/how-to-fix-fiddler-when-it-cant-set-the-proxy-on-windows-10/
A copy of the text, if the link will vanish one day:
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Type Windows+R and enter services.msc is the textbox.
Then look for WinHTTP Web Proxy Auto-Discovery Service, right click and select Properties.
Change the startup type to Automatic and then click on Start.
If for any reason you can’t change the startup type cause disabled, open regedit, go to HKLM\SYSTEM\CurrentControlSet\services\WinHttpAutoProxySvc and change start value to 2.