I encountered a similar problem to that described in this post but with version 7.5.7:
I was unable to save an email. It transpired that it was because no list was assigned to the email. After I assigned a list, I was able to save it. However, this does not seem normal, because a new email would not yet have a list assigned.
What happens if you update to the latest version, whcih is 5.7.9?
Sorry, I did not see your reply. I will check and let you know.