Random 500 error with Nginx reverse proxy Apache without error log

Hello all

I am getting some weird problem on my Nginx reverse proxy apache settings.

There is a random 500 error status code but nothing in the error log files (I can say it a about 20% of the traffic) as the site receives some thousand visits daily.

This is my Nginx config file

user www-data;
    worker_processes 4; 
    pid /run/nginx.pid;
    include /etc/nginx/modules-enabled/*.conf;
    
    events {
            worker_connections 4096; ## max clients = worker_processes * worker_connections
            multi_accept       on;
            use                epoll;
    }
    
    http {
            ##
            # Basic Settings
            ##
            sendfile on;
            tcp_nopush on;
            tcp_nodelay on;
            keepalive_timeout 65;
            client_body_timeout 12;
            client_header_timeout 12;
            send_timeout 10;
            types_hash_max_size 2048;
            server_tokens off;
    
            proxy_buffering on;
            proxy_buffer_size 16k;
            proxy_buffers 64 4k;
            proxy_busy_buffers_size 24k;
    
            include /etc/nginx/mime.types;
            default_type application/octet-stream;
            ## more tuning
            client_max_body_size 30M;
            client_body_buffer_size 12K;
            server_names_hash_max_size 1024;
            server_names_hash_bucket_size 112;
    
            client_header_buffer_size 2k;
            large_client_header_buffers 6 8k;
    
            ##
            # SSL Settings
            ##
            ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
            ssl_prefer_server_ciphers on;
    
            ##
            # Logging Settings
            ##
            #access_log     off;
            log_not_found  off;
            access_log /var/log/nginx/access.log;
            error_log /var/log/nginx/error.log;
            ##
            # Gzip Settings
            ##
            gzip  on;
            gzip_vary on;
            gzip_min_length 1000;
            gzip_comp_level 9;
            gzip_proxied expired no-cache no-store private auth;
            gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
            gzip_disable "MSIE [1-6]\.";
    
            ##
            # Virtual Host Configs
            ##
            include /etc/nginx/conf.d/*.conf;
            include /etc/nginx/sites-enabled/*;
    }

And this is an example of a site config file.

        ## Static cache settings #
    proxy_cache_path /tmp/site.com levels=1:2 keys_zone=temgo_cache_zone-site.com:15m max_size=10g inactive=60m use_temp_path=off;
    
    server {
        listen 80;
        root /var/www/site.com;
        index index.php index.html index.htm;
    
        server_name site.com www.site.com;
    
        location ~ ^(.*)\.(inc|inc\.php|sql|ht|htaccess)$ {
            deny all;
        }
        location / {
            try_files $uri $uri/ =404;
        }
    
        ## Include full Mod rewrite rules
        include /etc/nginx/ssl/site_global_rewrite.conf;
        
        set $no_cache 0;
        if ($request_uri ~* "(/login.php|/register.php|/user.php)"){
            set $no_cache 1;
        }
        location ~ \.php$ {
    
              proxy_cache temgo_cache_zone-site.com;
              proxy_cache_key "$scheme$request_method$host$request_uri$is_Mobile_Browser$cookie_DeviceType$cookie__gallery"; ## Based on Mobile Device
              proxy_cache_bypass $no_cache;
              proxy_cache_revalidate on;
              proxy_cache_min_uses 1;
              proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
              proxy_cache_background_update on;
              proxy_cache_lock on;
              add_header Strict-Transport-Security "max-age=47474747; includeSubDomains; preload";
              add_header Content-Security-Policy upgrade-insecure-requests;
              add_header X-Frame-Options sameorigin always;
              add_header X-Cache-Status $upstream_cache_status;
    
              proxy_pass http://127.0.0.1:8080;
              include /etc/nginx/proxy_params;
        }
    
        listen 443 ssl http2; 
        ssl_certificate /etc/nginx/ssl/site.com.crt;
        ssl_certificate_key /etc/nginx/ssl/site.com.key;
    }

Can someone please advise what can be wrong?

Regards, Jacques