murasame: Thank you for taking the time to answer so quickly. I don't think the problem is crossplatform related.
I've played on a number of servers with people obviously playing on windows and I've even hosted a server on my Linux distro where windows player were able to connect without a problem.
I've read somewhere else on GOG's forums that it could be related to how the GOG key works.
Here actually :
https://www.gog.com/forum/unreal_series/ut2004_cd_key_question/post2 Pityesz: Wow, so it is cross-compatible, this is a good info.
Yes, I have seen this too, but there was no problem with the default key. for me. Maybe this key is banned from some servers? I have tried again with three servers but I was able to play on it (I must note these was empty servers, I not wanted to disrupt others' game). You should try to request a new key, it solved the problem for others.
I really have no idea what is the matter. I heard that there is a problem like this but never encountered it. It is a tupid idea but try to reinstall the game before requesting new key. Delete everything that has connection to the game: registry keys for the GOG games, folders, and so on. Sometimes it solve the problems.
Reinstalling won't help as it is a case of banned key and the Gog key will sure remain the same.
Found it ! After trying to connect without success to another server, here is what I found on LamaPunter :
There is a note from Llama Punter Owners on this GUID.
71d5bc2350a20816a36b2cd2c7ee76ec is a GUID that belongs to GoG.com a fantastic website offering NO DRM games.
When you legally buy this game you get the same cd-key as everyone else who buys this. As you can guess with so many people using this cd-key it has triggered our piracy algorythm AND this cd-key is leaked all over the Warez Scene (torrent sites, etc) As you can see below here there are many bans of people cheating from this key. This key will be forever blacklisted end of story.
However you ARE in luck. We have talked with GoG and they said if you LEGALLY bought the game FROM THEM email them from the email you used to buy it with a cd-key request. They will give you a legitimate unused cd-key, but please give them 1-3 days to process it.