From what I can see at Logstash, TranslationNotifications is attempting to send emails to users without any email address set in their Preferences, obviously causing errors. The extension should not attempt to do that. See e.g. this case among others.
Addenda: Perhaps TranslationNotifications should be automatically unsubscribing these users when this happens, either immediately or after X unsuccessful attempts. An Echo notification could be sent telling the user why they have been unsubscribed as well.