Quantity in cart does not update correctly. Why ?

I don't know why the content of a cart cannot be updated (quantity cannot be changed, product cannot be erased). Any ideas where I should look to correct the problem ?
In other words, new products are added ok, but when a product is inside a cart, it cannot be removed, or the quantity changed.

Sounds like a file is out of date or a change has been made to the template that has broken the form tags etc.

Have you an URL to your site where this is happening?

http://www.nheo2.com/store/
Thank you for your quick reply.

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