The certificate chain is incomplete. (Chrome and Firefox only accept it because they've cached the Let's Encrypt intermediate from visits to other websites.)
https://www.ssllabs.com/ssltest/analyze.html?d=drive.nusalaska.com&hideResults=on
That only works in Apache 2.4.8 or newer. According to the site's Server
header, it's using 2.4.6. You need:
SSLCertificateFile /etc/letsencrypt/live/drive.nusalaska.com/cert.pem
SSLCertificateChainFile /etc/letsencrypt/live/drive.nusalaska.com/chain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/drive.nusalaska.com/privkey.pem