webaware,
My steps are more or less the same as yours. The problem is the preview link (or button) is redirecting to the URL in httpS. My site is run on http except the login and admin pages, so the static content like images or css cannot show properly on https. I can see the URL is in https when I mouse over the preview link of the unpublished post. If I remove the “s” on the preview URL, it works perfectly fine (of course it does =) ). I think the problem is all of the content is forces in https including the preview while it should be excluded or it should follow the site base URL.