Woocommerce not passing required fields on guest checkout

Somehow Woocommerce messed up the guest checkout at a client's website. It keeps saying that fields are required, though everything is filled in as should. I double checked if the page was being cached, but that was not the case. I am not using any checkout plugins and WC is up to date, so that is also not the problem. However it does work when the user is logged in, so it must have something to do with permissions. Does anyone know what causes this and how to fix it?

The website of my client is rubberdeal.com and is in Dutch. When you order you can see the issue (as the form is not passing the required fields check).

Thanks in advance.

via Chebli Mohamed