Secure pages and installing new modules (AJAX error)
I got this same error, but the cause was unrelated to secure pages, so it may not apply to your case.  Mine resulted from upgrading to Drupal 7.3 and having uncommented the two lines in the .htaccess file "To redirect all users to access the site WITHOUT the 'www.' prefix." When I commented those two lines, my module installer started working again.  So if you have those two lines in .htaccess uncommented, try commenting them out and see if that makes a difference.