When a quiz has feedback text or boundaries associated with it - but is marked as non-graded - it becomes impossible to edit the quiz because the fields which are marked as disabled (because mdl_quiz.grade = 0) have content which returns validation errors, but the feedback cannot be truncated without using developer tools to modify the DOM directly.
Here's a query to find the affected quiz feedback in your system (assumes a mdl_ prefix):
FROM mdl_quiz_feedback, mdl_course_modules AS m, mdl_modules AS m2, mdl_quiz AS q
WHERE mdl_quiz_feedback.quizid = m.instance
AND m.instance = q.id
AND m.module = m2.id
AND m2.name = 'quiz'
AND q.grade = 0
AND mdl_quiz_feedback.feedbacktext != ''
You can swap the SELECT to DELETE to mass "fix" this issue. I would propose that these fields be automatically truncated by simply not populating them when the quiz is not graded.