Hmm, well, thats about all I can really think of, sorry. Be sure that the key you are using isn't illegal.
You can try and delete the rtcwkey file. Then start wolfmp and go to options > enter cd key. Enter your key there and then press verify. If it is right it will say that the key is valid.
Just in case that did not work. Where did you get the game, did you ever exchanged your key with anyone?
Hope it works.