When I tried to log in to the forum for the first time, sso_login returned HTTP 422 with a simple error page. This kept repeating after several attempts. Then I logged in from a different browser successfully. Then logging in worked in the first browser, so I can’t reproduce it now. I don’t know if it was a temporary issue or something else, the only notable thing is that the Exercism session in the first browser was the original session after the recent signing up via GitHub.