1.xx - ERR_READ_TIMEOUT (Generally the HTTP request/response timed out when Akamai tried fetching content from origin. Default read timeout is 120s)
3.xx - ERR_READ_ERROR (Generally the TCP connection from Akamai to origin was successful but the origin did not send any (valid) HTTP response headers back)
4.xx - ERR_WRITE_ERROR (Usually happens when connection is closed by origin before Akamai could send the complete request)
6.xx - ERR_CONNECT_FAIL (When Akamai is trying to connect to origin, the origin or a firewall is actively refusing the connection - e.g. it sent a TCP RST packet)
9.xx - ERR_INVALID_URL (The Akamai Edge server does not have a configuration file associated for the host/URL visited)