The error: "2X SecureClientGateway failed to connect to HTTP server(s)." is returned when a user tries to connect to the HTML5 Gateway which is hosted in the DMZ.
Usually a Gateway deployed in a DMZ is setup as a forwarding gateway. In these instances, since a forwarding gateway essentially forwards all traffic to an internal gateway, just enabling the HTML5 Gateway on the Forwarding Gateway in the DMZ is not enough.
In order to be able to connect successfully, the HTML5 Gateway would need to be also enabled on the internal Gateway the Forwarding gateway is forwarding the connection to.