A bit more info
The owner and db entries were correct already here. I'm still having the problem, but did notice this message when I restarted psa:
[root@xxx init.d]# ./psa restart
===> Reading /etc/psa/psa.conf ...
PSA is down, perfoming full restart.
service mysqld does not support chkconfig
/etc/init.d/mysqld: Too many arguments (first extra is 'start').
Use --help to get a list of available options
An old version of the database format was found.\nYou need to upgrade the data format before using PostgreSQL.\nSee (Your System's documentation directory)/postgresql-7.4/README.rpm-dist for more information.
/usr/local/psa/admin/bin/httpsdctl start: httpd (pid 15642) already running
Starting Plesk: [ OK ]
Also, webmin is installed on this server and its working fine (in case thats relevant).