502 Error not understood on nginx server used with php

Question

I'm currently trying to switch from an Apache server to using nginx to be load resistant.

But I came across an error that I can not solve and I would be happy for your help, so: Most pages (php) work smoothly, Unexpectedly certain pages when I browse to them I get a 502 error Quote the error from the error.log logs

2022/01/25 18:51:57 [error] 848#848: *4 upstream sent too big header while reading response header from upstream, client: 147.234.64.35, server: XXXX.com, request: "GET /XXX.php?XXX=true&XXX=no&XXX= HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.4-fpm.sock:", host: "XXX.com"

But what I do not understand most is that when I enable error output by adding the value "ini_set ('display_errors', 1);" At the top of the code, it works smoothly without a 502 error

This is defined for me in the file / etc / nginx / sites-available / default

    server {
    server_name domain.com;
    root /var/www/html;

    index index.html index.htm index.php;

    location / {
        try_files $uri $uri/ =404;
    }

    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
     }

    location ~ /\.ht {
        deny all;
    }


    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/domain.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/domain.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot

}



server {
    if ($host = domain.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    listen 80;
    server_name domain.com;
    return 404; # managed by Certbot

}

I would appreciate any assistance or preparation Thanks

Answer 1

Try to add:

fastcgi_buffers 16 16k; 
fastcgi_buffer_size 32k;

or (if you use proxy_pass):

proxy_buffer_size          128k;
proxy_buffers              16 256k;
proxy_busy_buffers_size    256k;

If necessary change the values ;)