Civimail open tracking is not working for some mailings - inserts var/www/html into path for some mailings but not others
A workaround is to change the root civicrm token to an absolute path
(Credit due to Eli Lisseck for research and workaround solution)

Under Resource URLs, change CiviCRM Resource URL from [civicrm.root]/ to sites/all/modules/civicrm/

(Note that civicrm.settings.php has the following setting for sites that work correctly and sites that do not work correctly.
$civicrm_root = '/var/www/html/sites/all/modules/civicrm';)

So, while this appears to be a workaround, it is hiding the "root" problem.