After updating matomo to version 3.14.0, piwik-php-tracker is missing.
In /libs/PiwikTracker/PiwikTracker.php, there is following code:
if (!class_exists('PiwikTracker')) { require_once __DIR__ . '/../../vendor/piwik/piwik-php-tracker/PiwikTracker.php'; }

After the update, this file is missing and my whole script crashes.