After switching from HTTPS to HTTP (local development) WordPress may sometimes get stuck in the following error message:

Cookies are blocked or not supported by your browser.

The browser complains:

This Set-Cookie was blocked because it was not sent over a secure connection and would have overwritten a cookie with the Secure attribute.

The solution is:

1. Visit the https:// version of the site (it would error out as Connection Refused, or give you an SSL warning, whatever)
2. Clear the cookies while in the error screen.

Makes sense.