This error message is most commonly caused, as the name would suggest, by entering an incorrect username or password.
If you are confronted with this error message, we would suggest using your username (not your e-mail address) to log in - and if the error persists, to see if resetting your password through the website resolves the issue.
Note: We are investing reports that certain characters - for example ü - can trigger this error. We are investigating these reports, and ask that if you find a specific character which causes this error message, that you use the Contact Us link at the bottom of this support article to inform us of it. We apologize for the inconvenience as we resolve these bugs.