Ports Changing from Gateway Port 80 to IIS Site port

I am running a simple ASP.NET web application on a 2X application server and the initial page gets tunnelled correctly. After a successful login the URL automatically gets port 9090 appended to it and hence cannot be displayed to folks accessing outside our firewall.

This is bizarre...

http://sandbox.linium.net/s2g/login.aspx

USER: CM1
Pass: s2g

Login and you probably will not be able to get to the app because 9090 will be now part of the URL

PLEASE HELP!