In my previous post “Add to cart on some product pages with options dosent work” I stated the bug.
Today I finally found what caused it. If in a sidebar you put a widget with a dukapress product list, it creates a duplicate html form in a side bar with the same name as the main form with a product description. That confuses the function GetFinalPrice(). The solution will be to change a name for the product forms in a sidebar (e.g. add “sidebar_” prefix).