I had the same issue (Fedora 30 x86_64, kernel 5.2.9) and it turned out that being connected to a VPN was the problem. Apparently having a changed gateway address causes an “Error initializing network controller” error which I was able to see when I tried starting docker via sudo dockerd instead of sudo systemctl start docker.
I found the note here about the VPN being a possible problem, disconnecting immediately allowed me to start docker with systemctl start docker.