i think i found it:
$group_NAME = 'Attribute';
$group_query = new WP_Query( array( 'post_type' => 'acf-field-group', 's' => $group_NAME) );
$group_ID = $group_query->post->ID;
$field_query = new WP_Query( array( 'post_type' => 'acf-field', 'post_parent' => $group_ID) );
the second query returns me all available custom fields of the fieldgroup “Attribute”