A couple of things to look at:

1. Webserver logs. See if there is information in the webserver logs when the error happens. If nothing is showing up, increase the debug level to get more diagnostic information.
2. Use a packet capture utility like Wireshark to capture data at both the client and the server ends. This may capture some of the conversation that the logs may not.
3. This could be some type of firewall issue. Take a look at the firewall logs to see if anything is being blocked when connecting to the webserver from outside of the network.

Might want to give a look here, where there’s a similar problem reported.

If not, please give some firther information on your installation.