"Buy Now" button doesnt add product on first click

[SimoNtpf]

I have this problem:

 

On my site (www.minotaur.cz) when I click for the first time on "Buy Now" button in list of products in any category, the product is not added to shopping cart. When I click for the second time it work - and after that I am able to use the "Buy Now" button correctly.

 

Any idea how to fix it?

 

Thank you