Tracking on Civimail suddenly stopped working
That looks like a tracking image (it's 1x1 px and probably transparent), so you probably don't see it or get an error message.

Once after an upgrade, I somehow ended up with a space before the opening php tag in https://yoursite.org/sites/all/modules/civicrm/extern/url.php which broke tracking (can't have output before the header('Location: ' . $url);)