Euro sign incorrectly displayed sometimes

In my shop the euro sign is displayed incorrectly (€) for products with an option, but only on the price including the option, as shown on the following link:
hielspoorinfo.nl/webshop/product/40-baue...viscospot-hielkussen

Could you try to add the line below after the first <?php tag:
header('Content-Type: text/css; charset=utf-8');

in the file "option_price" of the view "product" via the menu Display->views ?

Perfect, issue is solved now, thanks!