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.