Weird.. The MySQL password of a certain database user changes after every Mysql restart. The only way to get it going again is by resubmitting the password using a seperate PhpMyAdmin install. Resubmitting the PW in Plesk breaks the login, and removing/adding the database-user, or adding a brand new one also doesn't help; the same problems occurs. I currently run a cron job which submits this users password every 15 minutes.. But that's not a good solution to the problem..