Thank you. I've changed as you suggested, but it still hasn't worked. When I clicked Next button, it just reloaded the page.

Here is my new nginx configuration file content:
location ~ \.(lock|conf)$ { deny all; }
location ~ pre_.+\.php$ { deny all; }
location ~ /[^.]+$ {
try_files $uri $uri/ $uri.html /index.php;
}
location ~ \.php(/.+)$ {
fastcgi_split_path_info ^((?U).+\.php)(/?.+)$;
fastcgi_param SCRIPT_FILENAME /var/www/siwapp/web$fastcgi_script_name;
fastcgi_param PATH_INFO $fastcgi_path_info;
fastcgi_param PATH_TRANSLATED $document_root$fastcgi_path_info;
}