I just tried to sign up for the forums using GitHub and GitHub rejected the OAuth request because of a redirect uri mismatch.
It redirects me to:
https://forum.matomo.org/auth/failure?message=redirect_uri_mismatch&origin=https%3A%2F%2Fforum.piwik.org%2Fc%2Fgeneral-questions&strategy=github
Which tells me that I “didn’t accept the request” or something like that