Nginx – Unexpected 404 response from php-fpm when file exists and is readable

I am running a simple PHP based site using nginx.
After a recent update of a number of system components, the site stopped working. When I try to access the site, I get a blank page with the text "File not found." The server logs tell me that

FastCGI sent in stderr: "Primary script unknown" while reading response header from upstream

and the PHP log contains

- - 25/Jan/2020:17:18:50 +0100 "GET /index.php" 404 - 0.151 2048 0.00%

The nginx config is as follows.

# configuration file /etc/nginx/nginx.conf:

user http http;
worker_processes  1;

events {
    worker_connections  1024;
}

http {
    include       mime.types;
    default_type  application/octet-stream;

    types_hash_max_size 2048;
    types_hash_bucket_size 128;

    sendfile        on;

    keepalive_timeout  65;

    # some server blocks elided

    include /home/myuser/www/com.mydomain.conf;
}

# configuration file /etc/nginx/mime.types:
types {
application/A2L                    a2l;
# lots of types elided so as not to exceed post size limit
}

# configuration file /etc/nginx/fastcgi.conf:

fastcgi_param  SCRIPT_FILENAME    $document_root$fastcgi_script_name;
fastcgi_param  QUERY_STRING       $query_string;
fastcgi_param  REQUEST_METHOD     $request_method;
fastcgi_param  CONTENT_TYPE       $content_type;
fastcgi_param  CONTENT_LENGTH     $content_length;

fastcgi_param  SCRIPT_NAME        $fastcgi_script_name;
fastcgi_param  REQUEST_URI        $request_uri;
fastcgi_param  DOCUMENT_URI       $document_uri;
fastcgi_param  DOCUMENT_ROOT      $document_root;
fastcgi_param  SERVER_PROTOCOL    $server_protocol;
fastcgi_param  HTTPS              $https if_not_empty;

fastcgi_param  GATEWAY_INTERFACE  CGI/1.1;
fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;

fastcgi_param  REMOTE_ADDR        $remote_addr;
fastcgi_param  REMOTE_PORT        $remote_port;
fastcgi_param  SERVER_ADDR        $server_addr;
fastcgi_param  SERVER_PORT        $server_port;
fastcgi_param  SERVER_NAME        $server_name;

# PHP only, required if PHP was built with --enable-force-cgi-redirect
fastcgi_param  REDIRECT_STATUS    200;
fastcgi_buffers 16 16k; 
fastcgi_buffer_size 32k;


# configuration file /home/myuser/www/com.mydomain.conf:
server {
  listen 80;
  server_name mydomain.com;
  # enforce https
  return 301 https://$server_name$request_uri;
}

server {
    listen 443 ssl;
    server_name mydomain.com;

    client_max_body_size 16m;

    root /home/myuser/www/com.mydomain;
    index index.php;

    location / {
        try_files $uri $uri/ /index.php?$args;
    }

    location ~ \.php$ {
        try_files $uri $fastcgi_script_name =404;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
        fastcgi_index index.php;
        fastcgi_pass unix:/run/php-fpm/php-fpm.sock;
   }
}

nginx and php-fpm are running as user http and the index.php file is accessible by that user:

-rwxrwxr-x 1 http http 1,7K  8. Nov 10:40 /home/myuser/www/com.mydomain/index.php

I have ascertained that $document_root and $fastcgi_script_name are correct by passing them as SCRIPT_NAME for testing.

What am I doing wrong?

Edit: This is indeed looking like a permission problem, but I still don't understand it. If I move the content of the site to /usr/share/webapps/ for a test, it works. Unfortunately, that is not an option for production use. With the files in their intended (original) location, I can run things like sudo -u http php /home/myuser/www/com.mydomain/test.php and get the expected result. What could prevent php-fpm (or the socket) from accessing those files? open_basedir is not set.