To you have your taxonomy field set to return Term Objects or Term ID? include
must be an array of Term IDs.
You can override the field setting and get just the IDs
<?php $zielgruppe = get_field( 'zielgruppe', false, false ); ?>
Your field may also be returning only a single value and you need an array
if ($zielgruppe) {
if (!is_array($zielgruppe)) {
$zielgruppe = array($zielgruppe);
}
....