I have solved the problem, the first nginx must be set as follows, then the second nginx belonging to zammad can run without changes and everything works.
Example configuration for the first nginx
server {
listen 80;
server_name example.com;
location / {
proxy_pass http://ziel-nginx-server$request_uri;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
}
}