Вот конфиг server { listen 80;

server_name slimus.mysite.com www.slimus.mysite.com;
return 301 https://slimus.mysite.com$request_uri;
}
server {
listen 443 ssl;
ssl_certificate /etc/letsencrypt/live/slimus.mysite.com/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/slimus.mysite.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

server_name slimus.mysite.com www.slimus.mysite.com;
index index.php index.html;

error_log /var/log/nginx/slimus.mysite.com;
access_log /var/log/nginx/slimus.mysite.com;

root /var/www/slimus.mysite.com/public;
location / {
try_files $uri /index.php$is_args$args;
}

location ~ \.php {
include snippets/fastcgi-php.conf;
fastcgi_pass unix:/run/php/php7.4-fpm.sock;
}

}
p.s есть еще конфиг defaut, может дело в нем?

Закреп пункт 3. Желательно приложить запрос курлом и лог на этот запрос