haproxy - unable to load SSL private key from PEM file

The order in which the cert and key files appear in the pem is important. Use the following to create the pem file.

cat example.com.crt example.com.key > example.com.pem

The problem I was running into on CentOS was SELinux was getting in the way. To test if SELinux is the problem execute the following as root: setenforce 0, then try restarting the haproxy. If it works, there is an SELinux problem. (You can re-enable SELinux now and try to fix the underlying problem with the command setenforce 1).

Since I have the certificates in the folder /etc/haproxy/certificates, the following command worked to get the right permissions on the files restorecon -v -R /etc/haproxy (depending on your OS and SELinux config this may or may not work).