I haven't checked a jail server in CentOS 6.4. You can use "Settings => Advanced Settings => Check Jail servers" as first test. Note that you must use a browser in a different machine to access the server and use the interactive console, If not, you get an "Permission denied" error in the console.
In any case check your server firewall. start removing the firewall adding it again when all are correct.
New Virtual Programming Lab (VPL) module