Unable to checkout in IE v11 in 3.20 - Products Disappear from Cart?

I have a new version of 3.20 of a store which has a problem with some products when you try to checkout.

This 'seems' to only affect Internet Explorer. Customer adds product to cart. Shows in cart, but when they click checkout the cart is empty? I have a video from the client showing this.

Annoying this is, it works fine for me and also works for the client in Firefox. I wouldn't have thought too much about it, but they just had a customer call with exactly the same issue?

I cannot reproduce it on our demo site. Everything works fine.

Ensure that your client has machine key specified in web.config. Also ask him to clear the browser cache (click F5)

Yes have a machine key and has cleared cache. I cannot reproduce, but he has sent a video showing the problem and also a customer called to say they had the same issue (Both IE11).

Also, must note it's only certain products...