If I add
add_filter('attachment_fields_to_save', 'wp_image_credit_save_credit', 10, 2);
function wp_image_credit_save_credit($post, $attachment) {
update_post_meta($post['ID'], WP_IMAGE_CREDIT_META_NAME, $attachment['credit']);
update_post_meta($post['ID'], WP_IMAGE_CREDIT_META_NAME_LINK, $attachment['credit_url']);
return $post;
}
it seems to work (though I cannot get the credit link to work).