Some of the previous posts seem to suggest that the problem arises primarily when highlighting has been applied in a previous version of Word. Kind of like trying to remove paint after it's dried, I guess. I confess my tests were limited--applying highlight
and immediately removing it--since I don't normally use this feature.
One thing you might try is whether you can remove highlighting with the Replace dialog. Click More to expand the dialog, then, in the "Find what" box, use Format | Highlight. In the "Replace with" box, you have to use Format | Highlight twice so that it
reads "Format: Not Highlight." You can either Replace All to remove all highlighting, or step through to find and replace just the specific highlights you want to remove.