I've looked around and seen several similar issues to this, but not exactly the same. I have civimail configured to send emails just find and the tracking url works initially, but ends in failure as it cannot find the civicrm.settings.php file in sites/default/
The problem may be that I have multiple sites running on a common installation of drupal. But, the sites folders are named the same as the virtual site (e.g., sites/example.com for example.com). It's not clear to me why Civicrm is failing to detect the proper path for the civicrm.settings.php file.
For reference: Civicrm 4.6.14, Drupal 7.42