Looking at your problem, I'm seeing the error anytime I enter the problem without submitting it. Given that and the "use of uninitialized value" error, my first guess is that there is something odd with a stored last answer for the problem. I've seen similar types of things when I've created a problem that had multiple answer blanks and then went back to fewer and had stored answers for problem blanks that didn't exist anymore.
To test this, I created a new set for myself which has that problem in it, and it did not show the error. So I next tried unassigning the set from you and reassigning it, which (I think) has cleared out the error. That doesn't mean that my diagnosis was correct, but is at least consistent.
Let us know if you see this again. There are others who know better where HTML::Scrubber is being used in the code, and they may be able to provide more insight on what's going on here.