We are in assignment submissions, viewing the list. Clicking in grade/update opens a popup window for processing teh Feedback for a single student. However, teh first element in the popup is a menu to select a group. This doesn't make sense, the popup window contains data for a single submission for a single student. Theer is no dneed to change the current group there. Even worst, if the group is changed you end with the whole submissions table within the popup window.

The error in in mod/assignment/lib.php, function display_submission() (and possibly in any overrided display_submission() method in plugins)

By line 807 these lines should be deleted:
if ($groupmode = groupmode($course, $cm))

{ // Groups are being used $currentgroup = setup_and_print_groups($course, $groupmode, 'submissions.php?id='.$this->cm->id); }

else

{ $currentgroup = false; }

That's all.