You can't register from within Voyager, but you can log in to an existing account (the button on the lower right of the adding an account screen). The reason you can't register a new piefed account in Voyager is because that functionality doesn't exist in the API yet.
I was able to log in to feddit just fine using curl, so it seems to be voyager-specific. Not sure if there might be an errant cloudflare challenge or other kind of proxy-level filtering that it might be getting caught up in.
Not sure if @aeharding@vger.social might have suggestions for how to help ID the issue or get a more descriptive error message.