It’s a small typo – I fixed it myself for now since I don’t have time to wait.
file: jigoshop_product_variation.class.php
line: 76
$sku = get_post_meta($this->variation_id, ‘_sku’, true);
should be
$sku = get_post_meta($this->variation_id, ‘sku’, true);
(in postmeta table meta_key values are ‘sku’ and not ‘_sku’)