I can get into the game completely fine, but whenever I attempt to play Multiplayer, it doesn't let me join and gives me this message "Login failed. Please check your username and password".

In the chat it says

To login type /login username password

I've done this with my username and password, yet I still can't get into the game... And plus whenever I type in my login like that, it doesn't even give any message (Whether the login is wrong or not, just nothing).

Very frustrating, cannot play.