If when trying to login to Plesk as an admin user, a popup says “Access Denied”, enter the following commands:
mysql -u admin -p`cat /etc/psa/.psa.shadow`
delete from lockout where login = ‘admin’;
NOTE: Plesk has an built-in security feature with account logins (basically only 3 login attempts are allowed). That is, if you get your password or username wrong 3 times, the server will lock you out.