I would prefer..
>Check the SMICM HTTP is properly defined with Hostname and port.
>Check from PO system if it's able to ping that ECC host
-If ECC is reachable then telnet the host with http port - :: telnet <ECCHOSTNAME> 80$$
-If No then another test is to get the IP of ECCHost and do telnet with IP this time :: telnet <IP> 80$$
For all NO contact your Network guys who manages the firewall and get the host/ip/ports allowed over network.
For Yes but still not connected then you have to check your configs in PO system. Check the hosts file in OS and you might have to check your PO systems host/ip and maybe maintain ECC. By PO's IP I mean is to check if the PO's ip is properly routed and configured in DNS. If routing of PO's is not correct then that can also be one cause.
Basically HTTP 404 response means - No Page found - Clearly anything from caller side is not able to reach/contact other side - simple block rule.
I feel this is network issue in initial message of yours so you can check these first.