In Plesk 9 Domains > click on the [domain] that's having issue > click on Web Hosting Settings.
In Plesk 11 in the My Subscriptions > click on the [domain] > click on the Change Hosting Settings tab.
In Plesk 12 Websites & Domains > Click on Web Hosting Access.
Change the password (the same password can be used, if desired).
Select OK and test the FTP connection.
2. Click on Start > Administrative Tools > Computer Manager.
3. Click on Local Users and Groups.
4. Click on Users.
5. Select the user account with the problem > Right click on it and select Properties.
6. Un-check Account Disabled.
7. Click OK and test the FTP connection.