Manage HTTP Access in Application Express 3.0
This is because the listener local access setting is being set to local only by default.
To set HTTP Access to allow remote connects:
SQL> exec dbms_xdb.setListenerLocalAccess (l_access => FALSE);
To set HTTP Access back to Local only:
SQL> exec dbms_xdb.setListenerLocalAccess (l_access => TRUE);
Another way to set this up is to login to APEX dashboard as a sys user and navigate to: Home > Administration > Manage HTTP Access (Tasks’ list on the right side of the page) and change the settings:
More information availble at Oracle: Oracle Application Express