The issue is due to Solaris 11.4 having two services by default that match system-log:
The issue is fixed in syslog-ng 6.0.19.
Download the installer of the latest version of syslog-ng PE 6 from our support portal.
The issue can be worked around by manually disabling the two system-log services before installing Syslog-NG. After installation Syslog-NG should run without further issues.