The certificate is not valid for urls community.elgg.org/*, i.e. "community" subdomain. This is not a browser issue. If you visit the community site directly, you get urls like elgg.org/* and the certificate should work without issues. Only if you follow old links that still have the old community subdomain it will fail. Best would be if the certificate would also be made valid for urls with the community subdomain. It has already been suggested but the admins most likely had no time yet to work on it.