Why is HAProxy unable to load certificates generated by letsencrypt?

The problem is that I was using the fullchain.pem file generated by letsencrypt. Instead, one should concatenate it together with 'privkey.pem' (also generated by letsencrypt) into a single .pem file.

I could only fix this by using cert.pem instead of fullchain.pem when concatenating.

cat cert.pem key.pem > haproxy_cert.pem