Your not trying to use the login/password from the forum, are you?
The in-game login is the name of your character, the password is on a per-character basis and you select both at character creation (registration from the games main menu). If you can't register a character name, then it's:
-used by someone else
-too short
-too long
-with illegal gliphs (@&%£$,'§¿¤ etc.)
Try another name.