And here is my code for Bootstrap 4.
I change the IDs name (#accordion) because I want to use multiple accordions on one page.
<?php if( have_rows('_accordion_zeile') ): $accordion++; ?>
<div id="accordion<?php echo $accordion; ?>">
<?php while ( have_rows('_accordion_zeile') ) : the_row(); $collapse++; ?>
<div class="card">
<div class="card-header" id="heading<?php echo $collapse; ?>">
<h5 class="mb-0">
<button class="btn btn-link" data-toggle="collapse" data-target="#collapse<?php echo $collapse; ?>" aria-expanded="true" aria-controls="collapse<?php echo $collapse; ?>">
<?php the_sub_field("_accordion_titel"); ?>
</button>
</h5>
</div>
<div id="collapse<?php echo $collapse; ?>" class="collapse" aria-labelledby="heading<?php echo $collapse; ?>" data-parent="#accordion<?php echo $accordion; ?>">
<div class="card-body">
<?php the_sub_field("_accordion_inhalt"); ?>
</div>
</div>
</div>
<?php endwhile; ?>
</div>
<?php endif; ?>
