Twice this month now. Site loads with a padlock in every browser, while curl and anything on Python fail with certificate verify errors:
curl: (60) SSL certificate problem: unable to get local issuer certificate
The server isn't sending the intermediate. Browsers cope by fetching it or having it cached. Everything else fails. With certbot, ssl_certificate has to be fullchain.pem and not cert.pem.
openssl s_client -connect example.com:443 -servername example.com -showcerts
If you only see one certificate block, that's it.