systemd-tmpfiles won't start
I had exactly the same problem today on my workstation - ubuntu 18.04.1.
Finally (searching systemd source code) I have figured, that somehow
my root folder = / ownership had been changed to me - the only user.
Simple chown root.root / solved the problem :)
... after some 2 hours of searching.
Encountered this problem as well.
To add to the answer above, you need to make sure that the whole path belongs to root, not just /. Somehow my /var folder got the owner of mysql (good job, mysql), and sshd was trying to create /var/run/sshd, which failed.