Hi csbubbles,
Some days ago, i resolved this issue by adding the SSLCertificateChainFile to my apache vhost, i use these settings:
SSLEngine on
SSLCertificateFile /etc/letsencrypt/live/domain.tld/cert.pem
SSLCertificateKeyFile /etc/letsencrypt/live/domain.tld/privkey.pem
SSLCertificateChainFile /etc/letsencrypt/live/domain.tld/chain.pem
I hope it helps.
EDIT: changed fullchain.pem to chain.pem (thanks to Osiris)