When signed out, click "Log in" at the top right of the site. Then click "Log in with GitHub" on the modal that appears. Nothing seems to happen. In console, I see:
15:09:40.043 Uncaught (in promise) TypeError: this.login.externalLogin is not a function
Thanks for letting me know.
Using the steps at Configure GitHub login for Discourse - admins - Discourse Meta
I have reset the client secret and successfully reproduced the issue registering a new user with a different GitHub account in a different browser (successful) - but after logging out I was unable to log back in using the 'Log in with Github' button either.
I have logged this at GitHub logins can create account but cannot login - support - Discourse Meta
Update - this was fixed a few minutes ago: Login via social account is broken for me - #4 by isaac - site feedback - Discourse Meta
I suspect we need to wait a short while for the fix be applied as we have free hosting thanks to the kind people at Civilized Discourse Construction Kit, Inc. see: https://free.discourse.group