Posted this in another topic, but:
We're having the same issue, and I've tracked down the cause.
The web browser is unable to communicate with
Why this is happening for you, I don't know, but for us it's because the method of connection for that site does not pass (Active Directory) credentials via our proxy server, so the proxy server rejects the connection.
The only solution for us is for Codeacademy to implement this.
All those sites listed are reachable, it's codecademy3428155-3020.ccterminalcloud.com that isn't.