What is the default browser used on your side (the one that opens the Fiddler login page)? Could you please try to change the default to Google Chrome, Firefox, or Microsoft Edge and let us know if the issue is still happening?
If the issue persist, send us the Fiddler log files to investigate the case further.
Additionally you could check your OS proxy settings alongside Fiddler Everywhere as described in the following article:
https://docs.telerik.com/fiddler-everywhere/knowledge-base/test-fiddler-network-access-on-macos (if your active adapter name is not successfully recognized by the script then try to rename the adapter name to something simple like MyLAN and restart Fiddler Everywhere)
Brave is my default browser.
I'm not going to spend any more time debugging this though.
I am also using Brave. However, I've noticed that the login flow is indeed not working correctly when using this specific browser. Brave seems to "remember" just one of my Google accounts and tries to connect Fiddler Everywhere with that very same account automatically. That would lead to a broken login process. I resolved the issue by temporarily switching the default browser to Google Chrome, authenticate with the proper account on the login selection screen (missing in Brave), and then using FE and switching back to Brave.
I choose "Sign in with Google" and it opens a browser window. I click my google user I want to sign in with. It authenticates and says I'm signed in and offers a link to "Go back to app". Nothing happens when I click that. Nothing happens if I don't click on that. Meanwhile, the app is still stuck in "Sign in via web browser".
Therefore, I can't sign in.