After we upgraded Mautic to v2.14.0, the
mautic:emails:send command started to report this error message:
mautic.NOTICE: Symfony\Component\Debug\Exception\FatalThrowableError: Type error: Argument 1 passed to Swift_Transport_SendmailTransport::send() must implement interface Swift_Mime_Message, boolean given, called in /var/www/mautic/vendor/swiftmailer/swiftmailer/lib/classes/Swift/FileSpool.php on line 169 (uncaught exception) at /var/www/mautic/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/SendmailTransport.php line 100 while running console command mautic:emails:send  
The messages are getting created in the “queue” … spool/default folder, but they are not getting sent … we are experimenting an issue as well with MySQL after the upgrade; it’s working slower and the MySQL process is using more CPU.
What could be the problem?