Just found a perfect solution by @Jpbe in a neighbour post: Mobile browsers security error
For nginx, it will be
ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
instead of
ssl_certificate /etc/letsencrypt/live/example.com/cert.pem;
ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem;
(added the solution here for a case somebody coming from Google)