vtxyzzy on "Can't get a WP query working"

Did you put $meta_query into your $args array? Like this:

if ($_POST['plaats'] != '') {
      $meta_query = array(
         'relation' => 'OR',
         array(
            'key' => 'city',
            'value' => $_POST['plaats'],
            'compare' => 'LIKE'
         ),
         array(
            'key'     => 'city2',
            'value'   => $_POST['plaats'],
            'compare' => 'LIKE'
         )
      );
   }
   $args['meta_query'] = $meta_query;
   query_posts($args);