Keep in mind that the username change can take up to 48 hours to fully go into effect-- this means that you may potentially have to wait a bit before the game registers the change to your account. One thing that I've noticed seems to cause a bit of trouble is trying to change the username and then changing the password and/or e-mail directly afterward. You may need to give the server time to work through each change at a time; if you find that you're still experiencing log-in troubles after 48 hours or so, choose one thing to change (e-mail, password, username, etc) and give it time to work.
Make sure your account is still active, as well. The server will often automatically removes accounts that are inactive (not having been logged into within 18 months).
Hopefully your problem gets resolved soon. c: