Hello,
I have the same problem since 2 weeks on my droplet with all php installations am trying to run.
I purged all nginx, php* packages then reinstall it again an set again all my vhost conf and it was working since yesterday.. all my wordpress sites. But, am just coming to get the same problem now.
My phpmyadmin new installation (after the previous apt purge) is a manualy installation with phpmyadmin archive: it brings me 502 too exactly like a phpinfo(); file . But any problem with Html site, on the sames vhost or not.

Bellow is the /etc/nginx/sites-available/ conf for one installation with a single index.php file

##BLOC DE SITE##
server {
        listen 80;

        root /home/orineltux/web/ubuntufacile/public_html;
        index index.php index.html index.htm;

        server_name ubuntufacile.tk www.ubuntufacile.tk;

        location / {
#               try_files $uri $uri/ =404;
                try_files $uri $uri/ /index.php?q=$uri$args;
        }

        location ~ \.php$ {
                try_files $uri =404;
                fastcgi_split_path_info ^(.+\.php)(/.+)$;
                fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;

                fastcgi_index index.php;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                include fastcgi_params;
        }
}

# Nouveau Bloc pour les parametres additionnels
server {
        # CONFIGURATION SSL
        listen 443 ssl http2;
        listen [::]:443 ssl http2;
        include snippets/ssl-ubuntufacile.tk.conf;
        include snippets/ssl-params.conf;
        server_name ubuntufacile.tk www.ubuntufacile.tk;
        root /home/orineltux/web/ubuntufacile/public_html;
        index index.php index.html index.htm;

        location / {
        ##try_files $uri $uri/ =404;
        try_files $uri $uri/ /index.php$is_args$args;
        }

        location ~ \.php$ {
                try_files $uri =404;
                fastcgi_split_path_info ^(.+\.php)(/.+)$;
                fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;

                fastcgi_index index.php;
                fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                include fastcgi_params;
        }

####POUR LETSENCRYPT############
        location ~ /.wel-known {
                allow all;
        }
}