Mark I'm having the same issue, including the behind-the-scenes rejected connections from codex.codeacademy.com in Safari and Chrome. I checked the certificate signatures and fingerprints, which match the main certificate; the only difference is that the top-level certificates aren't being presented from codex.codeacademy.com, just one intermediate. Aside from "just trusting" the certificate, I'm not sure what else to do.
Looks to have cleared up, probably from an earlier outage: https://twitter.com/Codecademy/status/699697815568654336
I'm not sure what the exact issue is, but @zystvan can you file this in the product issues queue?