Does this work:
<div class="postinner-h1">
<h1><?php echo single_cat_title(); ?> günstig online kaufen!</h1>
<?php
$term = get_queried_object();
if ( $term ):
?>
<p><?php echo get_field( 'short_description', $term->taxonomy . '_' . $term->term_id ); ?>
<?php endif; ?>
</div></div>