I hate to say it, but if you made a second account and it logs in just fine, something is wrong with other account. It doesn't meet the requirements to login properly (Special characters, too long of a name or password, etc.). I'm not sure how much trouble it's worth looking into; you may want to consider just playing on your second account that logs in just fine.
Don't fret over it; this has happened to many people, myself included. The first account I made had a password that was too long so I ended up creating and playing on my second account I made.
UPDATE: The new
password reset feature is now live. If you're still getting errors during login, try resetting the password using the e-mail address you registered your account with. Make sure it's less than 16 characters in length.