Ubuntu 16.04 - NGINX - LetsEncrypt SSL - 502 Bad Gateway for first time visiting the site. Everything works if you visited the site earlier.

I set up a web server for two Wordpress websites. Installed NGINX, PHP, Mariadb, certbot for SSL, etc…

The problem I´m facing is that if I visited the website earlier than Certbot´s config, I can access it normally, load different pages, admin panel, php works perfectly and everything is displayed and working fine. Nervertheless, if you are a new user that tries to access the website (https://liventplanning.com) you get a 502 - Bad Gateway error.

Here´s NGINX’s error.log:

2018/07/10 14:47:25 [error] 3425#3425: *1628 upstream sent invalid status "Service Unavailable" while reading response header from upstream, client: 37.9.113.120, server: liventplanning.com, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.0-fpm.sock:", host: "liventplanning.com".

And here´s NGINX´s …/sites-available/default file: (I removed all comments and most spaces, for simplicity)

server {
	server_name liventplanning.com;        
	root /var/www/liventplanning;
        index index.php index.html index.htm index.nginx-debian.html;
        location / {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                try_files $uri $uri/ =404;
                # proxy_pass http://localhost:8080;
                # proxy_http_version 1.1;
                # proxy_set_header Upgrade $http_upgrade;
                # proxy_set_header Connection 'upgrade';
                # proxy_set_header Host $host;
                # proxy_cache_bypass $http_upgrade;
        }

        location ~ \.php$ {
               include snippets/fastcgi-php.conf;

               # With php7.0-cgi alone:
               #fastcgi_pass 127.0.0.1:9000;
               # With php7.0-fpm:
               fastcgi_pass unix:/run/php/php7.0-fpm.sock;
        }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/liventplanning.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/liventplanning.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 {
       server_name megalaboratorio.com;
       root /var/www/megalaboratorio;
       index index.php index.html;
       location / {
               try_files $uri $uri/ =404;
       }
    listen [::]:443 ssl; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/liventplanning.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/liventplanning.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 {
        root /var/www/liventplanning;
        index index.php index.html index.htm index.nginx-debian.html;
	server_name www.megalaboratorio.com www.liventplanning.com; # managed by Certbot

        location / {
                # First attempt to serve request as file, then
                # as directory, then fall back to displaying a 404.
                try_files $uri $uri/ =404;
                # proxy_pass http://localhost:8080;
                # proxy_http_version 1.1;
                # proxy_set_header Upgrade $http_upgrade;
                # proxy_set_header Connection 'upgrade';
                # proxy_set_header Host $host;
                # proxy_cache_bypass $http_upgrade;
        }

        location ~ \.php$ {
               include snippets/fastcgi-php.conf;

               # With php7.0-cgi alone:
               #fastcgi_pass 127.0.0.1:9000;
               # With php7.0-fpm:
               fastcgi_pass unix:/run/php/php7.0-fpm.sock;
        }

    listen [::]:443 ssl ; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/liventplanning.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/liventplanning.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 {
    if ($host = liventplanning.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

        listen 80 default_server;
        listen [::]:80 default_server;

        server_name liventplanning.com;
    return 404; # managed by Certbot
}

server {
    if ($host = megalaboratorio.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

       listen 80;
       listen [::]:80;

       server_name megalaboratorio.com;
    return 404; # managed by Certbot
}

server {
    if ($host = www.megalaboratorio.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

    if ($host = www.liventplanning.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

        listen 80 ;
        listen [::]:80 ;
    server_name www.megalaboratorio.com www.liventplanning.com;
    return 404; # managed by Certbot
}

Any browser that has previously visited the site can use it perfectly, but if you are a new user, you get a 502 bad gateway.

Can someone point me in the right direction? I´ve been checking documentation, config files, forums and logs for a week and am still baffled by this. Thank you in advance for any help.