http working but not https

There was a report that one of our websites was down. Reported to PB, his reply was:

Found that http was working but not https

Then did the following:

root@ourserver:openssl s_client -connect our.domain.org:443 -servername our.domain.org

and got the error message

verify error:num=20:unable to get local issuer certificate

# comment out in  /etc/apache2/sites-enabled/our.domain.org-le-ssl.conf

# SSLCertificateChainFile /etc/letsencrypt/live/our.domain.org/chain.pem

and then restarted apache2