If ngninx test (/usr/sbin/nginx -t) gives you »
nginx: [emerg] a duplicate default server for 12.34.56.78:443 in /etc/nginx/conf.d/your-addon-domain.com_ssl.conf:4
nginx: configuration file /etc/nginx/nginx.conf test failed
as an error, it's because you have 2 or more _ssl.conf with "default_server" under /etc/nginx/conf.d/
Only 1 _ssl.conf (vhost) is allowd to have »
server {
listen 12.34.56.78:443 ssl http2 default_server;
…
}
Remove either all or remove all except 1 "default_server" inside your-main-domain.com_ssl.conf
Good luck.