Reading from the logs, what does this "...HTTP/1.1" 500 337985..." mean and how to fix it?

Question

Nginx

I am testing WebMoney payment on my server, debug logging level, both on the site and on the server.
Here is what I get in the nginx log in the access.log log when I try to send a test request from the WM wallet settings.

91.200.28.97 - - [29/Sep/2017:11:04:04 +0000] "POST / HTTP/1.1" 500 337985 "-" "Mozilla/4.0 (compatible; Win32; WinHttp.WinHttpRequest.5)"

As expected, WebMoney also tells me that the response returned an error 500. However, the error logs are empty, all that is this entry.
Error 500 means that the server is configured incorrectly, I don’t understand what I could have configured incorrectly, below are the server configs.
I don’t understand what the numbers after 500 are, and how does this help me in fixing the error?

# Default server configuration
server {
        error_log /var/log/error.log debug;

        listen  443 ssl http2 default_server;
        listen [::]:443 ssl http2 default_server;

        root /var/www/domain/public;
        index index.php index.html index.htm index.nginx-debian.html;
        server_name domain.com;

        # SSL settings, some parts are in nginx.conf. Let's Encrypt, cerbot package
        include snippets/ssl-domain.com.conf;
        include snippets/ssl-params.conf;
        location ~ /.well-known {
                allow all;
        }

        location / {
                try_files $uri $uri/ /index.php?$query_string;
        }

        # pass PHP scripts to FastCGI server
      location ~ [^/]\.php(/|$) {
                include snippets/fastcgi-php.conf;
                fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
                include fastcgi_params;
                fastcgi_read_timeout 999999999;
        }

        # deny access to .htaccess files, if Apache's document root
        # concurs with nginx's one
        location ~ /\.ht {
                deny all;
        }
}

# redirects simple port 80 to https + http2 + redirects IP to damain
server {
       listen         80 default_server;
       listen    [::]:80 default_server;
       server_name    domain.com 17***3;
       return         301 https://$server_name$request_uri;
}

Answer 1

log_format compression '$remote_addr - $remote_user [$time_local] '
                       '"$request" $status $bytes_sent '

$bytes_sent