Bob, Tom and others suffering with this,
I found out the following:
My business object group SystemUser had to BO's underneath: RegularUser and Members.
I got the error when I logged in as a member, however when I logged in as admin it worked. If you do the following it works in all cases.
Define a field say ChosenNewsletter in Members and when you save it, make sure AwareIm updates the other BO's in the group as well. Now, in my case,I needed to save the Newsletter I was working on, so I defined the following rule:
LoggedInSystemUser.ChosenNewsletter=Newsletter.ID
This will work for both admin and a member that is logged in.
Credits go to Tom, without whom I would not have been able to figure this out.