Nginx 502 Bad Gateway при загрузке файлов

Когда я пытаюсь загрузить файлы в свое веб-приложение на основе node.js, я получаю следующую ошибку:

2014/05/20 04:30:20 [error] 31070#0: *5 upstream prematurely closed connection while reading response header from upstream, client: ... [clipped]

Я использую здесь интерфейсный прокси:

  upstream app_mywebsite {
      server 127.0.0.1:3000;
  }

  server {
      listen 0.0.0.0:80;
      server_name {{ MY IP}} mywebsite;
      access_log /var/log/nginx/mywebsite.log;

      # pass the request to the node.js server with the correct headers and much more can be added, see nginx config options
      location / {
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_set_header X-NginX-Proxy true;

        proxy_pass http://app_mywebsite;
        proxy_redirect off;
    # web socket support
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
      }
   }

Это мой файл nginx.conf:

user www-data;
worker_processes 4;
pid /run/nginx.pid;

events {
    worker_connections 2048;
    multi_accept on;
}

http {

    ##
    # Basic Settings
    ##

    sendfile on;
    tcp_nopush on;
    tcp_nodelay on;
    keepalive_timeout 20;
    types_hash_max_size 2048;
    # server_tokens off;

    # server_names_hash_bucket_size 64;
    # server_name_in_redirect off;

    include /etc/nginx/mime.types;
    # default_type application/octet-stream;
    default_type text/html;
    charset UTF-8;

    ##
    # Logging Settings
    ##

    access_log /var/log/nginx/access.log;
    error_log /var/log/nginx/error.log;

    ##
    # Gzip Settings
    ##

    gzip on;
    gzip_disable "msie6";

    gzip_vary on;
    gzip_proxied any;
    gzip_min_length 256;
    gzip_comp_level 5;
    gzip_buffers 16 8k;
    gzip_http_version 1.1;
    gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript;

    ##
    # nginx-naxsi config
    ##
    # Uncomment it if you installed nginx-naxsi
    ##

    #include /etc/nginx/naxsi_core.rules;

    ##
    # nginx-passenger config
    ##
    # Uncomment it if you installed nginx-passenger
    ##

    #passenger_root /usr;
    #passenger_ruby /usr/bin/ruby;

    ##
    # Virtual Host Configs
    ##

    include /etc/nginx/conf.d/*.conf;
    include /etc/nginx/sites-enabled/*;
}

Есть идеи, как лучше отладить это? То, что я нашел, на самом деле не сработало (например, удаление хвостовой косой черты из моего proxy_pass