One of two things has happened (Vista 4.1.1):
1). A group created with Group Manager submitted a group assignment (not an individual assignment), and then the Group was deleted. It breaks the entire Assignment Dropbox for both Instructors and students.
2). A student submitted an assignment before withdrawing from the course. After the student drops, the Assignment Dropbox is unavailable to everyone else. For this one, re-enrolling the student again makes the Assignment Dropbox available. The student can then be denied access to prevent them from getting back into the course.
— as described on the listserve: firstname.lastname@example.org