after days of research, I found that the fault was simply in the pagination.
By changing the pagination code, I have made it work.
<?php
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$custom_args = array(
'post_type' => 'veranstaltungen',
'orderby' => 'date',
'order' => 'DESC',
'paged' => $paged,
'posts_per_page' => '10'
);
$wp_query = new WP_Query($custom_args);
if ($wp_query->have_posts()) : while ($wp_query->have_posts()) : $wp_query->the_post();
?>
<div class="accordion">
<div class="columns is-mobile is-flexend ">
<div class="column is-half">
<div class="columns is-mobile is-vcentered">
<div class="column is-one-fifth">
<i class="angle fa fa-angle-down fa-2x" aria-hidden="true"></i>
</div>
<div class="column is-four-fifths">
<?php
$categories = get_the_terms($custom_args->ID, 'veranstaltungen-category');
$cats=""; // set empty string as value
foreach ($categories as $i => $category) {
if ($i) echo ', ';
echo '<span>' . $category->name . '</span>';
}
?>
<div class="title">
<?php the_title(); ?>
</div>
</div>
</div>
</div>
<div class="column is-half">
<div class="postdate">
<p><?php the_field('tag') ?></p>
<p><?php the_field('datum') ?></p>
</div>
</div>
</div>
</div>
<div class="thepanel">
<div class="theimage">
<?php the_post_thumbnail(); ?>
</div>
<?php the_content(); ?>
</div>
<?php endwhile; ?>
<div class="columns is-mobile">
<div class="column is-3 is-offset-9">
<div class="pagination">
<?php
$nav = get_the_posts_pagination(array(
'mid_size' => 2,
'prev_text' => 'Zurück',
'next_text' => 'Vor',
'screen_reader_text' => 'A'
));
$nav = str_replace('<h2 class="screen-reader-text">A</h2>', '', $nav);
echo $nav;
?>
</div>
</div>
</div>
<?php endif;
wp_reset_query();
?>
Ahora funciona perfectamente. Gracias a todos