Hello i'm the whm user with root. This time i get the error from my customer when they login to cpanel and try to access to phpmyadmin direct on the icon it only show the login screen like this it must be a bug but not sure.
It's likely the MySQL password for the user was changed at some point and the account password was not. You can test this with other users, but I see no reason to believe it would a server-wide occurrence.