You must log in with the MySQL user name and password, not with the username of the client or an email user, etc.
When you create a database with ISPConfig, you have a database name like web1_db1 and a username like web1_u1. web1_u1 is the username you have to log in with.
When you log in with the user root, and no password is required, then that means that you didn't set a password with the mysqladmin command during the inititla setup. You should change that asap.