The newsletter remain in the "not sent" state. Cron Job is up and running and all mails work accordingly. newsletter subscribtion mails are sent as well.
magento/var/log/exception.log shows this obscure error:
main.CRITICAL: Error when running a cron job {"exception":"[object] (RuntimeException(code: 0): Error when running a cron job at /var/www/magento/vendor/magento/module-cron/Observer/ProcessCronQueueObserver.php:327, Error(code: 0): Call to a member function appendChild() on null at /var/www/magento/vendor/pelago/emogrifier/src/Emogrifier.php:1419)"} []
Any hints to solve the problem? thanks!