[PROBLEM]
Services on computer behind firewall are not reachable from outside, although iptables are properly routing ports through.
Also, this computer is perfectly available from the firewall and other "internal" IPs.
[SOLUTION]
Check if the unavailable computer has the correct gateway set.
In my case, I had a wrong route option set for THIS machine only, so all answers to requests routed through from outside could not find their way back to "the world".