Guests can't add products to cart

For some reason, if a customer is not logged in, they can't add their products to the shopping cart. I've unticked 'Force Login', but they still can't add products. The 'adding' loading button just keeps spinning.

I've tried adding a product to cart as a logged in member with no problems, but if I log out and then try to remove the product, the same thing happens - the spinning button with nothing else happening.

I've granted support access in case it's needed.