Products not getting into Cart

I’ve just updated to 2.5 this morning and have since noticed Cart issues I haven’t seen before.
When you add something to cart, the minicart gets updated only to get back to zero. If you try to go the cart page, it is also empty without any products added to it.

You can see this happening in this site: https://momentum.webipack.pt/

It isn’t producing anything in the logs.

Any tips please?

https://www.ads-software.com/plugins/woocommerce/