I never registered anything on the Bot web — just used the emulator.
However the v3 emulator (which is the current one) seems to behave differently than the old one I used for this article.
This means I need to configure ngrok, and possibly change some things to make it work. I’m not seeing an error like yours though; I am able to see a hit in the Functions logs when I try to connect.
What kind of auth are you using on the Function? I tried the default of adding a code in the query string, and just added this in the emulator config without bothering with username/password.