I saw this same thing happening on my installation after enabling port 80 tunneling. I think it might work for you if you disable this feature.
You can do this by doing the reverse of this:
http://www-1.ibm.com/support/docview.wss?rs=203&q=port+80+tunneling&uid=swg21090222&loc=en_US&cs=utf-8&lang=en+en
If you need port 80 tunneling I think you are in trouble - this is obviously a bug in 3.1. One can only hope it will be solved in the upcomming ST3.1 IF1.