WordPress seo woocommerce multilingual -> add to cart messed up

After installing WordPress Seo by Yoast, the following website malfunctions: http://www.mulliganproductstape.com
The english version works fine but the Dutch version is messed up. Take a look for yourself.
When visiting a product page and adding a product to my cart, I get a “no results found”-page. The product is added to the cart, but the add-to-cart-button doesn’t work like it should (you can compare to an english page..) because you get to see that “no results found” page…
I think the permalink structure is messed up.
I need to fix this quickly, this website is already launched and has many visitors. I hope you guys can help me out, it’s very urgent!

product page example; http://www.mulliganproductstape.com/sho … c/?lang=nl
hit “kopen” to see what I mean
Then you can go and compare it with the english version which works.

Thanks in advance!

http://wordpress.org/extend/plugins/wordpress-seo/