Nginx (Ubuntu 18.04) FastCGI sent in stderr: "Primary script unknown" while reading response header from upstream

Finally I made it work with the following set up... Here goes configuration which works for me.

Preconditions. Importnant!

• VPS congiguration: Ubuntu 18.04, Nginx + Php7.4-fpm + MariaDB + PHPMyAdmin. If you have the same package, fell free to copy-paste conf listings, he he
• Tested on Google Cloud instance, Amazon EC2 and Scaleway. In case of scaleway - some tweaks are required (like by default non root users can not run sudo command on Ubuntu 18.04 instance e.t.c.)
• If you are using different from 7.4 PHP-fpm ver make sure to reflect that in configuration
• This works for single site configuration with Letsencrypt SSL (there are many nice written instructures of how to set up Letsencrypt SSL with CertBot)
• BTW if you are using Cloudflare you might get redirect loop ERRTOOMANY_REDIRECTS Make sure to configurate SSL on Cloudflare side as well.

Corrections are welcomed

/etc/nginx/nginx.conf

user www-data;
# Check how many CPU cores do you have. There are 2 virtual cores are available on my VPS 
worker_processes 2;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;

events {
        worker_connections 768;
        multi_accept on;
}

http {

        #FastCGI cache settings
        fastcgi_cache_path /etc/nginx/cache levels=1:2 keys_zone=WORDPRESS:100m max_size=4g inactive=60m use_temp_path=off;
        fastcgi_cache_key "$scheme$request_method$host$request_uri";
        #
        fastcgi_buffers 8 16k;
        fastcgi_buffer_size 32k;
        fastcgi_connect_timeout 300;
        fastcgi_send_timeout 300;
        fastcgi_read_timeout 300;
        
        fastcgi_cache_background_update on;
        ##
        # Basic Settings
        ##

        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 15;
        types_hash_max_size 2048;
        server_tokens off;
        client_max_body_size 64m;
        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;
        default_type application/octet-stream;

        ##
        # SSL Settings
        ##
        ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
        ssl_prefer_server_ciphers on;

        ##
        # Logging Settings
        ##

        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        ##
        # Gzip Settings
        ##

        gzip on;

        # gzip_vary on;
        gzip_proxied any;
        gzip_comp_level 2;
        # gzip_buffers 16 8k;
        # gzip_http_version 1.1;
        gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;

        # DoS Protection set zone
        # limit_req_zone $binary_remote_addr zone=one:10m rate=3r/s;

        ##
        # Virtual Host Configs
        ##

        include /etc/nginx/conf.d/*.conf;
        include /etc/nginx/sites-enabled/*;

}

/etc/nginx/site-enabled/mysitename.conf

server {
#~~~forward to HTTPS~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name mysitename.com www.mysitename.com;

    return 301 https://$host$request_uri;
    access_log   /var/log/nginx/mysitename_http.access.log;
    error_log    /var/log/nginx/mysitename_http.error.log;
    # error_log off;
    # access_log off;
}

server {
#~~~HTTPS server settings~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#~~~BASIC settings~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
    listen 443 ssl http2 default_server;
    listen [::]:443 ssl http2 default_server;
    server_name mysitename.com mysitename.com;
    root /var/www/html/mysitename.com;
    index index.php;
    access_log   /var/log/nginx/mysitename_https.access.log;
    error_log    /var/log/nginx/mysitename_https.error.log;
    # error_log off;
    # access_log off;
    
#~~~SSL settings~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
    ssl on; 
    #~~~Certbot managed part~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    ssl_certificate /etc/letsencrypt/live/mysitename.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key //etc/letsencrypt/live/mysitename.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
    ssl_session_tickets off;
    #~~~Certbot managed part END~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    # HSTS (ngx_http_headers_module is required) (31536000 seconds = 1 year)
    add_header Strict-Transport-Security "max-age=31536000" always;

    # OCSP stapling
    ssl_stapling on;
    ssl_stapling_verify on;

    # verify chain of trust of OCSP response using Root CA and Intermediate certs
    # ssl_trusted_certificate /path/to/root_CA_cert_plus_intermediates;

    # replace with the IP address of your resolver???
    # resolver 127.0.0.1;
    
    add_header Content-Security-Policy-Report-Only "default-src https:; script-src https: 'unsafe-eval' 'unsafe-inline'; style-src https: 'unsafe-inline'; img-src https: data:; font-src https: data:; report-uri /csp-report";
#~~~SSL settings END~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~     

#~~~Caching rules~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
       # Enable caching
       set $skip_cache 0;
        
        #~~~Skip caching rules~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        
        # POST requests and urls with a query string should always go to PHP
        if ($request_method = POST) {
                set $skip_cache 1;
        }
        if ($query_string != "") {
                set $skip_cache 1;
        }

        # Don't cache uris containing the following segments
        if ($request_uri ~* "/wp-admin/|/xmlrpc.php|wp-.*.php|/feed/|index.php|sitemap(_index)?.xml") {
                set $skip_cache 1;
        }

        # Don't use the cache for logged in users or recent commenters
        if ($http_cookie ~* "comment_author|wordpress_[a-f0-9]+|wp-postpass|wordpress_no_cache|wordpress_logged_in") {
                  set $skip_cache 1;
        }

        #Don't chache for store/cart/my account/checkout pages
        if ($request_uri ~* "/store.*|/cart.*|/my-account.*|/checkout.*|/addons.*") {
         set $skip_cache 1;
        }

        #Skip cache for WooCommerce query string
        if ( $arg_add-to-cart != "" ) {
          set $skip_cache 1;
        }
 #~~~Caching rules END~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 #~~~Locations ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
        # Use cached or actual file if they exists, Otherwise pass request to WordPress
        location / {try_files $uri $uri/ /index.php?$args;}

        location ~ .php$ {
#                          try_files $uri =404;
                          fastcgi_split_path_info ^(.+.php)(/.+)$;
                          fastcgi_pass 127.0.0.1:9000;
                          fastcgi_index index.php;
#                          include fastcgi_params;

                          
####################
set $rt_session "";

        if ($http_cookie ~* "wc_session_cookie_[^=]*=([^%]+)%7C") {
                set $rt_session wc_session_cookie_$1;
        }

        if ($skip_cache = 0 ) {
                more_clear_headers "Set-Cookie*";
                set $rt_session "";
        }

        fastcgi_cache_key "$scheme$request_method$host$request_uri$rt_session";

        try_files $uri =404;


        include fastcgi_params;

        fastcgi_cache_bypass $skip_cache;
        fastcgi_no_cache $skip_cache;

        fastcgi_cache WORDPRESS;
        
        }

        location ~ /purge(/.*) {
            fastcgi_cache_purge WORDPRESS "$scheme$request_method$host$1";
        }

        location ~* ^.+.(ogg|ogv|svg|svgz|eot|otf|woff|mp4|ttf|rss|atom|jpg|jpeg|gif|png|ico|zip|tgz|gz|rar|bz2|doc|xls|exe|ppt|tar|mid|midi|wav|bmp|rtf)$ {
                access_log off; log_not_found off; expires max;
        }

        location = /robots.txt { access_log off; log_not_found off; }
        
        location ~ /. { deny  all; access_log off; log_not_found off; }

        # PHPMyAdmin access protection - please google of how to protect access to PHPMyAdmin. Some preliminary steps are required      
        location /customPHPmyadmin_path {
                        auth_basic "Admin Login";
                        auth_basic_user_file /etc/nginx/pma_pass;
        }

                
}