Changing URL results in access problem for home page only

For our public site we rotate two VMs, basically swapping the URLs when we’re ready to swap test and production versions. This has worked before. It’s not working today. Some new “feature” of 4.6.1?

In this case we have two URLs both resolving to the same public IP. We change the names then in two places: the Apache sites-enabled files, and two lines in wp-config.php:

define(‘WP_HOME’,’http://webtest.xyz.com’);
define(‘WP_SITEURL’,’http://webtest.xyz.com’);

So in the present case, http://webtest.xyz.com becomes http://webtest.abc.com (that is, we’re switching domain, not just subdomain). Then Apache is restarted. After that, the internal pages are available as always (e.g. http://webtest.xyz.com/wp-admin/). The Admin Settings page shows the new URLs picked up from wp-config.php. But the home page for the site by the new URL comes up blank in Firefox and Chrome — unless I substitute index.php with another index.php file (not WP) as a test.

If I bring up a virgin browser that hasn’t been to the old URL (in this case, Midori), the home page works. Looking at the logs Firefox and Chrome don’t even hit the server with the request. From the same (Linux) workstation w3m and lynx also connect fine to the home page. But something is gumming up the works for Firefox and Chrome. Is this something about how the home page is cached?