nginx uwsgi websockets 502 Bad Gateway upstream conexión cerrada prematuramente al leer el encabezado de respuesta de upstream

Encontré el problema.

Mi socket [runserver] (app.sock) debe apuntar debajo de upstream django y mi socket [wsserver] (django.sock) debe apuntar debajo de location /ws/ así:

upstream django {
    server unix:/opt/django/app.sock;
}

server {
    listen 80 default_server;
    charset utf-8;
    client_max_body_size 20M;
    sendfile on;
    keepalive_timeout 0;
    large_client_header_buffers 8 32k;

location /media  {
    alias /opt/django/app/media/media;  
}

location /static {
    alias /opt/django/app/static;
}

location / {
    include /opt/django/uwsgi_params; 
}

location /ws/ {
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_pass http://unix:/opt/django/django.sock;
        proxy_buffers 8 32k;
        proxy_buffer_size 64k;
    }
}