An "invalidresponse" message will appear if a student has, at any point in a forum, replied to a post having a "Display period" that, at the time of grading, has expired. 

1. Post to a forum as an instructor. Specify, in the post that the "Display period" should end in ten minutes.
2. Within those 10 minutes, have a student reply to that post.  Also have the student post anywhere else within the same forum.
3. As an instructor, return to the forum after the display period has ended, click the "Grade" button, and select that student's name.  You'll be met with an "invalidresponse" message indicating "Invalid response value detected." 

It seems to me that the instructor should be able to use the Whole Forum Grading interface to see (and grade) posts even if they were made in reply to a forum post whose "display period" has ended by the time of grading.  Minimally, I would expect that the instructor should be able to use the interface to grade posts made elsewhere in the forum.