What method are you using to view the mashup? Are you clicking View Mashup in the mashup builder and receiving this error, or are you navigating to the url for the mashup?
Does it work from the machine running ThingWorx? If so, have you checked for any firewall settings that may be blocking access to the port from outside the server.
Are you sure you're entering the correct port in the url?
Please check these things and if you still have issues, provide further information so we can assist.