@artlady I think codex.codecademy.com is only used on the older courses, which are being phased out. Are you able to access any of the other old courses, such as Ruby?
I don't understand why only codex.codecademy.com is causing an error for you, however. When I visited https://codex.codecademy.com and inspected the certificate, this is what I saw:
They're using a wildcard certificate, there, the same one used on www.codecademy.com. Is it really only that one subdomain giving you an error? That sounds pretty odd to me.
If you visit codex.codecademy.com itself, do you get any errors? In Firefox, I know that sometimes you'll be presented with a warning that the page is insecure, but you can add it's certificate and trust it anyway in some cases. Are you able to do that here?