In case you are using ESET Apache HTTP proxy, it is probable that connections to port 8883 are blocked but it is not clear whether this is the issue. Could you verify configuration of:
AllowCONNECT 443 563 2222
in httpd.conf? It is possible that enabling this port 8883 will helps AGENT to connect successfully.
In case direct connection to EPNS servers is not possible, multiple alternatives, including port variants (443,8883) are tried to ensure that connection is made even when configuration is not possible.