One more possibility I can think of, then.
E) If you're using a non-english keyboard, FS2_open assumes a different key layout from what you actually have. As such, while the standard keys (A-Z, 0-9) are generally the same, anything else might potentially give difference results in-game versus in normal use. And since the password only shows as ******* when typed, you can't really tell. So if you are on a non-english keyboard and are using non-alphanumerical characters in your password, try registering a new account with a strictly alphanumerical password.