there's seems a regression here, I'm no more able so login to piwik/matomo via github, keeps on cycling for new account etc :/ - seems same as #11590
You can try logging in via password instead as it goes to the same account as long as you use the same E-Mail address.
But at least for me Github Login is still working.
Forum login should work. If it doesn't, please contact us directly or create a new issue.