Sorry for the delay. The issue here is indeed related to your proxy. I can’t exactly say what it is, but one of domain name, protocol (http or https) and port are getting in the way.
Passkeys verify that the frontend and backend both run on the same domain, protocol and port. If one of these is a mismatch, you’ll get an error like this.
In the Rails CLI, can you try this:
DiscourseWebauthn.origin
and compare it to the URL that you use to access the site in the browser? The two should match.