This error shows up a lot when I launch kmail from cli. But this plugin does not show in the configure "settings plugins" window. There is a text to speak tab in the New Email Notifier setting for akonadi agents, but nothing has ever been enabled here as I discovered this while going through each and every plugin trying to find the one causing the error message.
text-to-speech is not developing by kmail it's based on qtspeech => qt you need to report error to qt thanks