Why doesn't submitting a form via https work when nginx is configured to only serve connections via https?

Question

Django

Nginx is configured like this:

# Default server
server {
    server_name _;
    return 404;
}

server {
  listen 80 default_server;
  listen [::]:80 default_server;
  server_name example.com www.example.com;
  if ($scheme = http) {
        return 301 https://$server_name$request_uri;
    }
}

server {
    listen 443 ssl;
    keepalive_timeout 60;
    server_name example.com www.example.com;
    charset utf-8;
    access_log /dev/stdout;
    error_log /dev/stdout info;

    # ssl cache settings
    ssl_session_cache shared:SSL:20m;
    ssl_session_timeout 180m;

    ssl_stapling on;
    ssl_certificate /letsencrypt/live/example.com/fullchain.pem;
    ssl_certificate_key /letsencrypt/live/example.com/privkey.pem;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_prefer_server_ciphers on;
    ssl_ciphers ECDH+AESGCM:ECDH+AES256:ECDH+AES128:DHE+AES128:!ADH:!AECDH:!MD5;

    location /uploads/ {
        autoindex on;
        alias /uploads/;
    }

    location / {
        # prevent access for some bots
        if ($http_user_agent ~ (libwww|Wget|LWP|damnBot|BBBike|java|spider|crawl) ) {
            return 403;
        }

        proxy_pass http://localhost:5000;
        proxy_set_header Host $host:$server_port;
        proxy_set_header X-Forwarded-Host $server_name;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

But with this setting, submitting the form does not work, the following error is returned:

400 Bad Request. The plain HTTP request was sent to HTTPS port

Answer 1

As a result of the discussion, we found out (thanks to Alexey Ten ) that the problem was in the application itself. As it turned out, it was necessary to enable support on the django side (version 1.8):
specify in settings:
SECURE_SSL_REDIRECT = True
SECURE_PROXY_SSL_HEADER = ('HTTP_X_FORWARDED_PROTO', 'https')
and add security middleware:
django.middleware.security.SecurityMiddleware

Answer 2

localhost:5000 >> https://localhost:5000 change