I just realised that https://wordpress.meta.stackexchange.com/ doesn't have a proper SSL certificate set as the site kind of broke when using HTTPS Everywhere. The main site is fine, just the meta site is affected.
The certificate served is only valid for:
*.stackexchange.com, stackexchange.com, meta.stackexchange.com, stackoverflow.com, *.stackoverflow.com, stackauth.com, sstatic.net, *.sstatic.net, serverfault.com, meta.serverfault.com, superuser.com, meta.superuser.com, stackapps.com, openid.stackauth.com, *.meta.stackexchange.com