Just upgraded to 2.0. Add to cart is now broken.

After upgrade, when I use the custom HTML "add to cart" link, the cart does not update, the mini cart does not update, and I am redirected to the empty cart URL with a message "The cart is empty."

I am at a loss - any advice?

Do you have edited the views "product / show_default" or "product / quantity" ?

If it is, try to save you custom views, remove the changes in HikaShop (dustbin in the view listing) and see if it's working fine.
Then adapt your code for the newest version.