I’ve just been updating an old site and discovered the same problem. I found that if I deactivate the quiz plugin, the Divi Builder works again. Once I’d finished editing I reactivated it and although the builder wasn’t visible again, the quiz and page worked okay. Not ideal but I was still able to do the job.
Incidentally, I’m not sure if its relevant, but for me the only page that was affected in this was was the page which contained the quiz.