Configuration issue. Path to the certificate file was missing a slash.
1 - Check the error in the output for what file it cannot find.
2 - Check the file exist and ensure the path to the file is absolute and correct in the syslog-ng configuration file(Default location is /opt/syslog-ng/etc/syslog-ng.conf). For example in one case it was the pem file for the certificate was missing the /.
3 - Restart the daemon:
systemctl restart syslog-ng