Why do I get HTTP Error 504: Gateway Timeout on accessing public IP over Android

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http://serverfault.com – Question: When I start a webserver on my machine (on port 80), and then try to access it via my public IP, I get HTTP error 504 (Gateway Timeout). Why ? This only happens when I am connected to the internet via Android. When I connect via normal ADSL, everything works fine. Is there any way to enable this ? If Android was a router, I'd say I need to port-forward. But it's strange I get HTTP 504 then. Anybody knows ? (HowTos)