You may need to set SSLCertificateChainFile. Example:
$ sudo grep "ssl:" -A 3 /etc/ood/config/ood_portal.yml
ssl:
- SSLCertificateFile /etc/letsencrypt/live/ondemand.osc.edu/cert.pem
- SSLCertificateKeyFile /etc/letsencrypt/live/ondemand.osc.edu/privkey.pem
- SSLCertificateChainFile /etc/letsencrypt/live/ondemand.osc.edu/chain.pem