I was busy troubleshooting an issue with one of our clients web server yesterday. The webserver is using different SSL port when accessed externally.
I configured port forwarding and opened the port on the firewall but when I tested accessing the application through web browser it gave me error (Refer to the screenshot on the left). Hhhhmmmnnnn. Something fishy about this. I enabled logging on the firewall and point to a syslog server and then initiate a connection. It seems that the traffic/request was forwarded to the webserver but it gave me [Error 400: Bad Requst] You sent a plain HTTP request to an SSL-enabled server port. I was very confident that there was nothing wrong in network configuration perspective. My last resort was to verify the webserver configuration. I logon and saw the configuration (Refer to screenshot below).
Now, the issue was on the IIS configuration. The vendor who setup the webserver erroneously configured the port. The port should be configured on SSL port not on TCP port. Whhhhewwww! Thus, it took me two hours figuring out where was the issue. This is hilarious!