"Could not find a valid session key." seems to have regressed.

On a brand new Wordpress install, I get this when submitting any admin form.

"Sorry but we are not able to provide this at the moment.
We can't load the requested web page. This page requires cookies to be enabled in your browser settings. Please check this setting and enable cookies (if they are not enabled). Then try again. If this error persists, contact the site adminstrator for assistance.

Site Administrators: This error may indicate that users are accessing this page using a domain or URL other than the configured Base URL. EXAMPLE: Base URL is http://example.org, but some users are accessing the page via http://www.example.org or a domain alias like http://myotherexample.org.

Error type: Could not find a valid session key."

Environment (Server): Nginx, HHVM, MariaDB. All in Docker containers.
Environment (Browser): Windows 7, Chrome 42.

Related: CRM-9483, CRM-13564, CRM-13021.

I have technical skills to debug, and patch, but am not familiar with the codebase.