Plone installation as created a few minutes ago by a current buildout.coredev: the creation of a Plone site via the ZMI fails due to https://github.com/zopefoundation/Zope/issues/490
See https://github.com/plone/Products.CMFPlone/pull/2727 for a fix to the old patch that added the "add plone site" button. You can currently only create a site from the frontpage at localhost:8080
Thank you for your response.
I have created Plone sites by manually fixing the broken action urls via the "firefox" "inspector" (thereby verifying that the only problem is the broken ZMI add).