TYPO3 / direct_mail: The body of “Symfony\Component\Mime\Part\TextPart” must be a string or a resource (got “bool”)

Initial situation The TYPO3 extension direct_mail does not send newsletters anymore. In the scheduler and in the system log the error message “Core: Exception handler (CLI): Uncaught TYPO3 Exception: The body of “Symfony\Component\Mime\Part\TextPart” must be a string or a resource (got “bool”). | TypeError thrown in file /…/vendor/symfony/mime/Part/TextPart.php in line 52″ Workaround Edit the file … Read more