You need to do this from the Reports > datasheet view and right click a story to show the Summary Form. I only get this error when I've edited one of the rich text fields, not when I edit the story name or pick something from a dropdown.
For example, all of the views remember your last setting. So, if your datasheet (or report ) always gives you an error but works fine for others, the problem could be with the dates you had selected last time the report was run. The dates you entered last time are "remembered" by the system as part of your user settings.
