I have received this error whenever I try to generate large reports with my App in PHP.
It does not end the generation and ends the connection by not downloading it.

Nginx.conf
user nginx;
worker_processes auto;

error_log /var/log/nginx/error.log debug;
pid /var/run/nginx.pid;


events {
worker_connections 2048;
}

http {
server_names_hash_max_size 512;
server_names_hash_bucket_size 128;
include /etc/nginx/mime.types;
default_type application/octet-stream;

log_format main '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';

access_log /var/log/nginx/access.log main;
# access_log off;

sendfile on;
#tcp_nopush on;
client_body_buffer_size 800M;
client_max_body_size 400M;
keepalive_timeout 120;
keepalive_requests 2048;
#gzip on;

# Enable Cache Support
# proxy_cache_path /data/nginx/cache/app levels=1:2 keys_zone=STATIC:10m max_size=1g;
proxy_cache_path /data/nginx/cache/app levels=1:2 keys_zone=app:10m max_size=2g inactive=60m use_temp_path=off;

real_ip_header X-Forwarded-For;
real_ip_recursive on;
proxy_buffers 16 16k;
proxy_buffer_size 16k;
proxy_connect_timeout 300000;
proxy_send_timeout 300000;
proxy_read_timeout 300000;
send_timeout 300000;

include /etc/nginx/conf.d/*.conf;

server {
listen 8081;
server_name 10.100.0.239 10.100.0.187 10.100.0.27 10.100.0.230;
large_client_header_buffers 4 128k;
location / {
stub_status;
}
location /server_status {
stub_status;
}
}
}

location.conf
server {
listen 80;
server_name teste.exemple.com.br;
gzip on;
gzip_vary on;
gzip_comp_level 4;
gzip_types text/plain text/css text/javascript application/x-javascript text/xml application/xml;
gzip_min_length 1400;
server_tokens off;
add_header X-Content-Type-Options nosniff;
proxy_hide_header X-Powered-By;
client_header_timeout 5s;
client_body_timeout 5s;
location / {
proxy_pass http://prd_exemple_teste;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header Host $host;
proxy_http_version 1.1;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_connect_timeout 60s;
proxy_send_timeout 60s;
proxy_read_timeout 60s;
send_timeout 60s;
}
}