If this is an external vRO, the port should be 8281 and not 8283 (for Control Center only).
Yes, 8281 is the TCP port for the main vRO service.
OK we resolved problem by using HOSTNAME(fqdn) without _ "underscore"
Previous i had:
Now we have:
and looks like is working and error 503 not appeared.
Sometimes one wrong character can cause an error in vRO. It's good to know in the future how to solve a problem like You had. Thanks for reporting how You dealt with this problem. Good tip for the future.