Hi,
The add the cart button doesn't work when https:// protocol is active, because of this request:
Blocked loading mixed active content "www.forster-winzer.de/index.php?option=com_virtuemart&nosef=1&view=cart&task=addJS&format=json&lang=de&quantity%255B%255D%3D1%26option%3Dcom_virtuemart%26view%3Dcart%26virtuemart_product_id%255B%255D%3D113&_=1385544337084"
Because the website is trying the make the http:// call even though the protocol is https://. Therefore it gets blocked (which is normal). You shouldn't be using https:// in the category page though, this doesn't make any sense and not necessary. But if you insist to use the HTTPS:// in all the pages; you may try to include this line:
var $live_site = '://www.yourdomain.com';
or
var $live_site = 'https://www.yourdomain.com';
into configuration.php file.
OR- use the .htaccess force redirection method.
But i would suggest you to use SSL, only in checkout and account pages. To do it; through the Virtuemart Backend, enable this setting:
Enable SSL for sensitive areas (recommended)
and leave everything to Virtuemart. Virtuemart will automatically redirects to https:// in the checkout and account pages, which are the pages where you actually need to use SSL.
Regards.