Problem with Options

The following problem exists on my site for a while. Problem probably exists for a while already, so may not have been introduced with the latest upgrade. No changes have been made by myself in this product. See hielspoorinfo.nl/webshop/product/40-baue...viscospot-hielkussen for the product in error.

The problem is with a product for which I have set up an option, whith 3 values. The default option has an additional price, the other 2 don't. When customer order the default chosen option, all goes fine. However, when they choose the cheaper option, they get a message that that option is not available.
The base product is put in the basket though, and they can proceed to checkout, but the chosen option is not shown in the order-details, so I have to call the customer to ask them which one is chosen.
I have attached a screenprint of the error, and the way the product is set up.

All quantities are set to Unlimited.

Thanks, that fixed it.