Deluge torrent client says "no incoming connection"

Question

Deluge is not connecting in my Ubuntu 12.04, whereas Transmission is.

I have no firewall restriction/forward configured as far as I know, the port deluge uses (60233) is successfully tested by deluge, and by http://canyouseeme.org/:

Success: I can see your service on 94.109.X.X on port (60223) Your ISP is not blocking port 60223

The error Deluge displays varies: mostly no incoming connection, sometimes Error: Host not found (authoritative) -for ubuntu.com tracker-, often timeout (simultaneously to no incoming connection).

As this seems to be no network issue (since Transmission connects & downloads just fine), but more a deluge specific problem: where should I look for problem? And how to solve it?

Solution

I had the same problem in the past. I DMZ'ed my PC through the router and it worked.

Look into your router for DMZ.