"Start quiz" button not working / form is not submitting.

The "Start quiz" button generated from which is in the form quiz_start_quiz_button_form() should use type "submit" not "button". Such as

function quiz_start_quiz_button_form($form, &$form_state, $node) {
  $form = array();
  $form['#action'] = url("node/$node->nid/take");
  $form['button'] = array(
    '#type' => 'button', // <=============== This should be changed to "submit"
    '#value' => t('Start quiz'),
  );
  return $form;
}

If you override the theme_button() to use button tag instead of input the form will not submit.