Hi,
Thank you for the access.
We found the issue and fixed it by changing the line:
if($cart_type == 'cart' && $this->params->get('show_cart_proceed',1)) echo $this->cart->displayButton(JText::_('PROCEED_TO_CHECKOUT'),'checkout',$this->params,$url_checkout,'');
to:
if($cart_type == 'cart' && $this->params->get('show_cart_proceed',1)) echo $this->cart->displayButton(JText::_('PROCEED_TO_CHECKOUT'),'checkout',$this->params,$url_checkout,'window.location=\''.$url_checkout.'\';return false;');
in the file "showcart" of the view "cart" via the menu Display>Views.