I add your line to default file and i have a same error in browser, now my code look like this:

server {
    listen [::]:80;
    return 301 https://$host$request_uri;
}

server {

    listen [::]:443;
    server_name domukasi.pl www.domukasi.pl;

    location / {

      proxy_set_header        Host $host;
      proxy_set_header        X-Real-IP $remote_addr;
      proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header        X-Forwarded-Proto $scheme;
      proxy_set_header X-Forwarded-Proto https;

      # Fix the “It appears that your reverse proxy set up is broken" error.
      proxy_pass          http://127.0.0.1:3000;
      proxy_read_timeout  90;

      proxy_redirect      http://127.0.0.1:3000 https://domukasi.pl;
    }
  }

I checked my syntax, and everything is okay. Is there any other way to get rid of this error?

I also checked a listening on ports 80 and 443, here are the results
sudo lsof -i :80

COMMAND   PID     USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
nginx    6346     root    6u  IPv6 15423161      0t0  TCP *:http (LISTEN)
nginx    6348 www-data    6u  IPv6 15423161      0t0  TCP *:http (LISTEN)
nginx   12503     root    6u  IPv4   468507      0t0  TCP *:http (LISTEN)
nginx   12548 www-data    6u  IPv4   468507      0t0  TCP *:http (LISTEN)

sudo lsof -i :444

COMMAND   PID     USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
nginx    6346     root    7u  IPv6 15423162      0t0  TCP *:https (LISTEN)
nginx    6348 www-data    7u  IPv6 15423162      0t0  TCP *:https (LISTEN)
nginx   12503     root    7u  IPv4   468508      0t0  TCP *:https (LISTEN)
nginx   12548 www-data    7u  IPv4   468508      0t0  TCP *:https (LISTEN)

sudo netstat -ltnp | grep -w ’:80’

tcp6       0      0 :::80                   :::*                    LISTEN  

sudo netstat -ltnp | grep -w ’:443’

tcp6       0      0 :::443                  :::*                   LISTEN