Access denied for user 'test'@'localhost' (using password: NO)

My login attempts fail for a new user account that I have created, and I have spent several hours learning how authentication is handled by MySQL, with no progress.

Everything is on the same server:

• MySQL 5.7
• phpMyAdmin 4.9
• Ubuntu 18.04

The end result doesn’t change, regardless of whether I:

1. Run the query (as root)
2. Open a browser on a different machine and run a simple PHP connection script

Here’s what I have tried from phpMyAdmin (logged in as root):

flush privileges;
create user 'testuser'@'localhost' identified by 'password!';
grant * on inventory to 'testuser'@'localhost';
flush privileges;

While logged into Ubuntu via ssh: mysql -u testuser -p mysql -u testuser -h 127.0.0.1 -p mysql -u testuser -h localhost -p mysql -u testuser -h <the-actual-hostname> -p

When attempting to log in, depending on whether I used 'testuser'@'%' or 'testuser'@'localhost' I’ll get one of two errors:

Access denied for user ‘testuser’@‘%’ (using password: YES) Access denied for user ‘testuser’@‘localhost’ (using password: YES)

To rule out this being a permissions issue, I assigned the user all global permissions, but the error message didn’t change, so I reverted the change.

Running ‘’‘select host,user,plugin,authentication_string,account_locked from mysql.user where user=testuser;’‘’ shows:

• There are no users with blank usernames in the table mysql.user
• The password doesn’t have special characters
• I’ve tried localhost, 127.0.0.1 and the actual hostname itself

Just to show another way I’m trying to connect, I have a LAMP stack running and navigate to this php page in multiple web browsers:

<?php
$dbhost='localhost';
$dbuser='testuser';
$dbpass='password';
$dbname='inventory';

$conn = new mysqli($dbhost,$dbuser,$dbpass,$dbname);
if($conn->connect_error) {
	die("Error: Couldn't connect: ".$conn->connect_error);
}
?>

I’ve tried deleting the user and re-creating again after having confirmed that there’s no blank usernames or duplicates of this user.

/var/log/mysql/error.log only shows the exact same error message.