This error is typically accompanied by a MySQL database connection error, usually mysqli_connect(): Access denied for user 'username'@'localhost' (using password: YES).
In my experience, any MySQL error occurring with the automatic login when redirecting from cPanel to PHPMyAdmin can be resolved by resetting the cPanel user's password. The issue is that the password is out of sync between cPanel and MySQL for the user.
Log into WHM → List Accounts
Click the plus sign next to the account and enter a new, strong password and Save.
This will sync the password to MySQL for the user. You should now be able to log into cPanel as the cPanel user and click "PHPMyAdmin" to be redirected and successfully authenticated to PHPMyAdmin.
If you have any trouble, please open a support request with your KnownHost support team and we'll be happy to help! :)