Your connection is not secure, how can you secure it then?

Question

Nginx

When trying to access phppgadmin.domain.com; gives a certificate error.

server {
         listen 443 ssl http2 default_server;
         listen [::]:443 ssl http2 default_server;

        root /var/www/domain/public;

        index index.php index.html index.htm index.nginx-debian.html;

        server_name domain.com;

        include snippets/ssl-domain.com.conf;
        include snippets/ssl-params.conf;
        location ~ /.well-known {
                allow all;
        }
         if ($scheme != "https") {
                return 301 https://$host$request_uri;
         } 

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

     location ~ [^/]\.php(/|$) {
                include snippets/fastcgi-php.conf;
                fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
                include fastcgi_params;
        }

        location ~ /\.ht {
                deny all;
        }
}

# PhpPgAdmin
server{
        server_name     phppgadmin.domain.com;
        root            /var/www/domain/phppgadmin;
        index index.php index.html index.html

        access_log      /var/log/phppgadmin/access.log;
        error_log       /var/log/phppgadmin/error.log;

        location ~ \.php$ {
                include snippets/fastcgi-php.conf;
                fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
               include fastcgi_params;
        }

        location / {
                allow   18*****
                deny    all;
        }
}

I could go to PhPPgAdmin for a couple of days, now it gives an error:

Your connection is not private

Attackers might be trying to steal your information from phppgadmin.domain.com (for example, passwords, messages, or credit cards). Learn more
NET::ERR_CERT_COMMON_NAME_INVALID

--2017-09-20 10:39:01--  http://subdomain.domain.com/
Resolving subdomain.domain.com (subdomain.domain.com)... 178.62.247.93, 2a03:b0c0:2:d0::349e:1001
Connecting to subdomain.domain.com (subdomain.domain.com)|178.62.247.93|:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2017-09-20 10:39:01 ERROR 403: Forbidden.

// ping

[email protected]:/etc/nginx/sites-available$ ping subdomain.domain.com
PING subdomain.domain.com (178.62.247.93) 56(84) bytes of data.
64 bytes from 178.62.247.93 (178.62.247.93): icmp_seq=1 ttl=64 time=0.066 ms
64 bytes from 178.62.247.93 (178.62.247.93): icmp_seq=2 ttl=64 time=0.122 ms
--- subdomain.domain.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms

Link to a similar question
The error, as I understand it, is that it redirects to a subdomain with a certificate from the domain. I do not understand why then it worked for a couple of days.
Can you please tell me how to write the correct redirect then?