Can't add products with variants to the cart

Hi Guys,

i'm currently implementing perch shop functionality into an existing project. Everything works really nice, but as soon as i add variants to a product, i can't put it into the cart anymore. (i use the example template for it).

Here is my diagnostics report:

Perch: 2.8.34, PHP: 5.5.11, MySQL: mysqlnd 5.0.11-dev - 20120503 - $Id: bf9ad53b11c9a57efdb1057292d73b928b8c5c77 $, with PDO
Server OS: WINNT, apache2handler
Installed apps: content (2.8.34), assets (2.8.34), categories (2.8.34), perch_shop_orders (1.0.12), perch_shop_products (1.0.12), perch_shop (1.0.12), perch_members (1.5)
App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_members', 'perch_shop' );
PERCH_LOGINPATH: /perch
PERCH_PATH: C:\xampp\htdocs\giftbox\perch
PERCH_CORE: C:\xampp\htdocs\giftbox\perch\core
PERCH_RESFILEPATH: C:\xampp\htdocs\giftbox\perch\resources
Image manipulation: GD
PHP limits: Max upload 20M, Max POST 8M, Memory: 128M, Total max file upload: 8M
F1: 6a33f95eca3667f9e0c39bf5ca2980fe
Resource folder writeable: Yes
HTTP_HOST: heidelbergbox.hgl
DOCUMENT_ROOT: C:/xampp/htdocs/giftbox
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php

any idea where the problem could be?