JQuery Multistep Progress-Bar: next_fs Problem

Question

we use a multi-step form with a progressbar. For some reason, the progressbar doesn't add the class="active" to the 2nd step after the first button click, but jumps right to step 5.

Can you see my error?

$(".next").click(function() {
  if (animating) return false;
    animating = true;

    current_fs = $(this).parent();
    next_fs = $(this).parent().next();

    //activate next step on progressbar using the index of next_fs
    $("#progressbar li").eq($("fieldset").index(next_fs)).addClass("active");

The page this is used: https://quiz.purevisionmethod.com/app/blurry-vision/

Thanks for your help!