Taskconsole stopped working

It looks like you have a deleted user.
Regarding the code, maybe we should add the username from the exception to at least know the account that is failing.

                // There is no account by that name.  Log this as an exception.
                else {
                  throw new MaestroGeneralException(t('Unknown account name (@account) identified when attempting a notification.', ['@account' => $accountName]));
                }

@Vladimir MS:
Can you determine if this is indeed a user which has been deleted and if altering the template to send notifications to the appropriate replacement user is sufficient?

Placing in Postponed status to determine which is the most appropriate course of action here.