Not sure why its only on this page, but maybe its because there are more than one pages worth of replies and one post has been flagged as an answer?
If you click the All Responses or Answers buttons at the top then part of the page becomes almost invisible like there is an almost opaque div overlay.
Only that page Gerard, but I wonder if the combination of more than one page of replies with one answer, and that the answer is on page 2 of the replies, is causeing the problem overlay div to appear. I can see the div in Firebug, its white with a 0.75 opacity. Harry is right, a refresh removes the problem.