More often than not, an incorrectly formatted user name or password is the cause. User names and passwords are case sensitive. This means that if you used the password 3r4s5H6B8a, then the H and B in the password need to be uppercase and the r, s and a need to be lower case. If you used a space or a special character (!@#$%^&* etc.) as part of your user name or password, the system will truncate it - remove it - from the login/password. So if you login name was John Doe, it is now JohnDoe. If your password was 7935@$&32, your password is now 793532.
It is also important to remember that changing your user name in-game does not change your login name. If you use JohnDoe when you create your account, no matter how often you change your user name under "Edit Account" in-game, your login name will still be JohnDoe.