If you get the above error then it means that there is some problem with the mysql privileges which you can easily fix by running the below commands from shell :
[email protected][#] vi /etc/my.cnf
and add this line :
save and quit.
Restart the mysql service on the server.
[email protected][#] /etc/init.d/mysql restart
this allows you to login back to plesk
and now to fix the mysql privileges execute the below command:
[email protected][#] mysql_fix_privilege_tables --user=admin --password='yourpassword' --verbose
now restart mysql service for the final time
That's all you should now be easily able to change the Plesk admin password from the Control panel.