Similar to this one: 921252

In case you set up a network source then upon boot syslog-ng fails to start
because network.target has not been reached yet.
Once you log in and start it manually it starts up just fine.

I had to add network.target to the systemd service file once I copied it into
/etc/systemd/system

[Unit]
...
After=var-run.mount network.target
...

There is nothing extraordinary in syslog-ng's conf. Just add a network() source
to any given log path, then reboot the system.