I don't recall you have to connect to the open servers, it's just a local gateway to connect to other players. However, the usual culprit is either a firewall (software) or a router (hardware). If you're sure your ports are open I'm not entirely sure. I could try it myself.
Edit: I seem to have issues myself both connection and apparently my Undeworld key is invalid.
Anyway, there's a possibility open servers were connected to Ascaron's servers but I remember reading something on the official forum about being able to if you connected to a back up server or perhaps an unofficial run by some fans.
I would recommend running a VPN (Tunngle, GameRanger, Hamachi) and then playing via LAN. This requires organisation so you can't really play with randoms.
This seems to be a good page: http://www.sacredwiki.org/index.php5/Sacred:Sacred_Public_Servers