We created a new instance of WordPress which automatically included the Root Relative URLs plugin. Turning it off made even the basic theme break. We sleuthed it out with Liquid Web. We're the first to bring it to their attention, we suspect not the last.
The URL in the db didn't match the ones in the General >Settings, so somehow it's not updating the db properly.
In PHP MyAdmin, we went wp_options in the db and updated both URL entries to match what it was supposed to be.
That allowed Pro to load and allow editing with the preview.
Thanks for your patience and assistance to narrow down the issue,.