Its a weird second layer of security on the server. On character creation in Diamond days, you had to create a login on character creation for character authentication, to prevent anyone logging in with your character.
With EE, that is not necessary, as the CD key is now logged with Steam, however the issue remains due to the carry over of the old Diamond security system. In your case, you've logged in and thankfully you've not had the issue of others where your login wasn't working, and logged in immediately. However, despite getting past Steam's security, you've now encountered Diamonds old security system (which remains in place) and have to use the login you created way back when. To log in, use @login (password without brackets) if you can remember what you used. If you cannot, a DM can come in and reset your password if you go to the Generic Requests thread on the forums and log who you need the help with, if you can't get a DM in game.
