You need to have a control user who can access the table User in database mysql.
You need to grant permissions to a new member you make under root to do this. A possible user is made under root with
GRANT ALL ON mysql.User TO phpmyadmin@localhost
IDENTIFIED BY 'phpmyadmin'
Please choose a better password.
Then, update the config.inc.php file as mentioned in the last post.
Now you can login.