You can also try logging in to that site in Safari and then using the Connect button (you should be already logged in, thus skipping the broken login form).
There are two separate sessions: the one for the site loaded in the webview, and the one loaded by the Connect button, which uses a special Safari session (via ASWebAuthenticationSession). So, even if you’re logged in to the site in the DiscourseHub webview, you won’t be logged into it once you click Connect.
So, the suggestion is this: open Safari and login to the site. Then, switch to DiscourseHub and click Connect. Hopefully then you will be in the Authorize step (because Safari’s logged in session means you are logged in to the site for the special view invoked by Connect).
Oh, wow. What content blocker are you using? Maybe it’s catching some class or ID in our login form? IIRC, the board game site does not have any social logins.