Plesk Onyx fix file permissions

The error message

Unable to open the directory: Unable to find the directory /var/www/vhosts/example.com/httpdocs/dir/: filemng failed: filemng: opendir failed: Permission denied System error 13: Permission denied

Login as root to your server and type

plesk repair fs yourdomain.com

You can also use this command for fixing subdomain permissions.