Yeah, after upgrading to 16.04 smbd did not start because I still had security=share in the config file. That question is answered here Samba does not launch after upgrading to 16.04.
But I have a different question. Where are the error messages from samba? Starting failed, and I got told to look at systemctl status smb.service or journalctl -xe. Both told me the process exited with an error, but didn't log any error message from smbd itself.
Is systemd THAT unusable?
Edit: or: please point me to any way i could have seen that error on my system instead of just googling for "smbd doesn't start after upgrade to 16.04".