Docker plugin can't create docker resource

I'm trying to configure foreman docker compute resource, and get an error
when clicking "test connection"

Unable to save
Expected([200, 201, 202, 203, 204, 304]) <=> Actual(503 Service Unavailable)

this is with the url

any ideas?

CentOS 7 / Foreman 1.8 (just upgraded from 1.7)
Installed docker plugin from repos.