Hello,
This error is most commonly thrown when one of the users set to receive the email does not have an email address configured. Could you check the users (TOC > Administration > Users) to make sure they all have an email address?
If you run into older forum posts like this (more than a year old) in the future that don't answer your question, I'd recommend making an entirely forum post instead of responding with a comment to the older post. It will increase the visibility of your question to everyone in the community.
Chris