Live previewing of themes not working in the customizer

Because I have been using a theme which allows users to configure it using its own pages, I haven’t been using the theme customizer at all, so I don’t know if it ever worked previously. Because WordPress is going to require theme developers to use the customizer, I thought I should try it out on some new themes on a copy of our site that sits on a development machine. When I did so, the left-hand panel appears and allows me to change settings, but the live preview panel merely shows a never-ending loading symbol.

Here’s what I have tried so far without luck:

• Disabling all plugins
• Removing the entire plugins directory as described in another forum posting (I have since restored it)
• Switching to a different web browser that has no installed extensions
• Changing the wp-theme (and its subdirectories), so their group membership is “www-data” and changing the group permissions for those directories to read, write, and execute
• Modifying wp-config.php by changing define(‘DISALLOW_FILE_EDIT’,true); to be false.
• Changing my .htaccess file to allow access to wp-config.php

Since I used svn to install WordPress core at my site, and continue to use it to update the core installation, I tried using svn again to reinstall the core; it said I was at the current revision. So I tried going back to a previous revision (4.1) updated the database, then switched back to the current revision (4.2.2) and updated the WordPress database again. I now had a fresh install of the WordPress core. Without enabling any plugins, I tested the live preview. Nothing worked. Because another forum posting mentioned that wp-super-cache was the cause of their difficulties, I note that at one point I had installed this plugin, but it was disabled and its directory was removed long ago.

As to my working environment, I have both a production environment and a development environment. Both use apache as their web server, and both use SVN to implement core updates. In both I update and install plugins by using sftp to move the files to my machines, then log on to the machines using a terminal to move them into place.