I was trying to get the error you are seeing occur without success. I ran DataAssist and based the pages off a template. I then ran Security Assist and I tried specifying one of the DataAssist pages as the redirect page. It said the page was based off a template and I needed to select a template from the wizard to do this.
I then selected the template from the wizard and made this selection again and it took without issue. I'm wondering if this is an issue with the template file you are using. Please post back with this file so that I can do some more testing on it to see if I can ge the same result.
Have you had the other error occur more than once? Did this happen for you when you were getting the first error? Please post back with some specific steps that you are taking when you get this error so that we can check on this also.
