I've got another idea...
Check whether your web server is listening on both ipv4 and ipv6 ports. I had a similar problem (access from LAN worked, from the outside it didn't) before and I noticed that my web server only listened on the computer's ipv6 address. Disabling ipv6 (so that the webserver switched to listening on the ipv4 address) worked.
Run on your web server
and make sure you have both tcp and tcp6 lines like this:
Code:
tcp 0 0 0.0.0.0:myport 0.0.0.0:* LISTEN
tcp6 0 0 :::myport :::* LISTEN
or at least the tcp one. If you only have the tcp6 line, you could try disabling ipv6.
AFAIK there is no firewall by default on Ubuntu Server boxes, so this problem simply cannot happen because of that. I'm guessing this ipv6 issue is what causes your problems.
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