Problems when adding to cart from product listing page

I just upgraded this site from v 1.5.1 to 1.5.4 using the usual process of new folder - check it all - etc.

I seem to have found an odd error though when adding products to the cart from the product listing page

As you can see on this page there are more than 1 product listed :
http://www.mcfigures.com/assassins-creed/assassins-creed-ser-3

If you add only 1 of those products to your cart you will see at the top in a red error message the following :
PLEASE CORRECT THE FOLLOWING:
... QUANTITY UNITS ERRORS - MCFARLANE ASSASSIN'S CREED SERIES 3 - AH TABAI ADD TO CART:

One error per product with a 0 quantity on the page.

Can someone please help me figure out why this is happening. It only started with this upgrade.

Thank You!
Tina

When the quantity of zero, it can't add a product to the cart.

Also when I mouse over (hover) the "add selected items to cart" the button jumps around, you may need to modify your css.

How did you come to host your site using zc 1.5.4? Did you use a process similar to: http://www.zen-cart.com/entry.php?3-How-do-I-rebuild-my-site-on-the-new-version-instead-of-upgrading

Did you start with a fresh install? If an upgrade, from what version? Where did you obtain the fileset? Does the problem occur when temporarily using the default zc template?