The error it's kicking back is simple, and the cause even more so. You've been using your cpanel name to login to mysql instead of going through the database wizard and creating a new user.
This works fine until you update your cpanel password, because the user it's having problems accessing the database with is your cpanel name. Find the MyBB config file and upate the password within it; I'd also highly recommend going to the Databases section of cPanel and creating a new user just for MyBB with all permissions, which will prevent the forum from melting down every time you change your password
