Nevermind, i’m dumb… solved it:
$kategorie = get_field('kategorie_eintragen');
$the_query = new WP_Query( array(
'post_type' => 'touren',
'posts_per_page' => -1,
'post_status' => 'publish',
'orderby' => 'date',
'order' => 'ASC',
'tax_query' => array(
array(
'taxonomy' => 'touren-categories',
'field' => 'slug',
'terms' => $kategorie
)
)
));