Ok. As a security measure you should password protect your phpmyadmin directory. You can do this from cPanel. Also use a directory name that is hard to guess .. like domain.com/mysql1abc/
I'm not working with Webmin but you can take a look here brightlemon.com/blog/password-protecting-directory-webmin-1
I always put my phpmyadmin into random named folder, that i simply bookmark in my browser for quick access. I also always deny access by IPs to anyone for that folder and whitelist only my IP
Protect the directory with an htaccess / password that is different from you phpMyAdmin password. Also make sure you phpMyAdmin version is up to date.
Yeah, would also prefer that, just double safe it with the htaccess and the normal phpmyadmin login. But be sure that you use strong passwords with numbers, letters and special chars
If you have SSH access to your server, make sure PhpMyAdmin is available on 127.0.0.1 only. SSH to use machine and use port forwarding to connect to PHPMyAdmin. If you don't have SSH access but have a static IP at work or home, restrict access to PhpMyadMin to this IP.
I always put phpmyadmin into the random named folder,and update the password weekly.You need to consult to web Hosting provider to change the password of your PhpAdmin.
I would guess someone is reading your sites config.php file and getting phpmyadmin details, most likely with a shell script. Do you share the server? or maybe there is a compromised site on the server, is your MySQL pass unique, eg. not the same as the hosting panel login? If you think its brute force install CSF firewall and configure it to IP ban after 3 failed login attempts.
If you run your own dedicated server or VPS then contact me Else, contact your provider and explain the issue, make sure they change all your passwords/usernames and firewall settings!