phpMyAdmin gives #1040 Cannot log in to the MySQL server error after a few weeks

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http://serverfault.com – I am running MySQL and phpMyAdmin (under Apache) on a CentOS VPS, everything works fine, but after a few weeks services using the database start having issues and all users except for root get the '#1040 Cannot log in to the MySQL server' error message in PMA. Disk space and RAM are not the problem, the VPS is running with 2GB of RAM of which 300mb is being used and only 19% used disk space. Any ideas? (HowTos)