I hope to have a corrected version of this problem in the CVS later today.
There is a couple of difficulties impeding the effort to correctly mark students' scores in this case. Once we have the problem patched, if a student (or professor, acting as a student) submits her/his answer the system will indicate that the negative answer is correct. However, once the set is closed the student obviously can't submit the answer again to get credit, and in no case is the professor able to submit a student's problem for credit. Both of these behaviors are, I think, the most appropriate ones in general circumstances. Furthermore, my experience is that a student who has an answer marked wrong may then try another (very wrong) answer, with the result that the last submitted answer is not necessarily the correct one.
The result of this is that the effort of checking to see if a student should get credit may also require going back through her/his past answers. And I don't know of any way to automate the checking of all 60 students' current answers to see which of them had a correct answer that was not marked right.
If it is not appropriate to simply give all students credit for the problem (or to delete it) in this case, I would have any students who submitted a correct answer that was marked wrong contact me individually. This adds a small amount of work for them (they have to check their answer and e-mail me), and eliminates the substantial work required to go through each student to manually check whether they have a correct answer.