I'm sorry you keep receiveing that error message. I am more than happy to help troubleshoot this with you. Are you putting any type of special characters in the header? Does your subject line or from name have any unusual characters? I would try just clearing out the subject line and the from name and seeing if that lets you exit the header options. If you are still struggling please let me know the name of the email you are working on and I will be happy to take a look at it for you.
I had this same problem. It was fixed by my logging out of Constant Contact and then logging back in.