Just for kicks, if you run /arachni/bin/arachni_web -D -o 184.108.40.206 without the service, does the application UI appear? If so, my gut is leaning towards a daemon/user permissions issue, but that's just a guess.
Admittedly systemd or initd setup is not my area of expertise. However, if you're able to run the exact same command as the service but when you invoke the service it works, I'm inclined to think of the issue as one of two things:
1) Permissions: Does the daemon running the service have the permissions that it needs?
2) Race Condition: Are there dependencies that are available once the system is fully booted that might not be available when the systemd or initd service is called?