Plesk will lock out accounts if multiple failed attempts to login are registered. The default is to lock accounts for 30 minutes after 3 failed attempts. Attempting to login with an account that has been locked out will result in an “Access Denied” pop up – even if the password used is correct.
If you cannot wait for the account to become unlocked, you can manually unlock it by removing the database entry from the lockout table in psa.
Login into your server via ssh and run this command:
root@plesklinux root]# mysql -uadmin -p`cat /etc/psa/.psa.shadow` psa -e 'DELETE FROM lockout