Shopping Cart 'Update Cart' not working

We're running a slightly older (patched) 1.21 release and noticed that when you click Update Cart with multiple items in the cart, it goes to the products_general_info oage , which I since fixed with a patch from the archived support forum that then redirects to the products_info page.

But it isn't updating the products in the cart at all as it should be.
It seems to be trying to redirect with products_id being an array.

/index.php?main_page=product_info&products_id=Array

We're hoping to upgrade to the latest release very soon, but as the cart is in use on a production site, we have to schedule the upgrade and so forth.

Any ideas where I would look to try and track this down? Filenames would be useful.. It's submitting to index.php?main_page=shopping_cart&action=update_product, but then seems to be redirecting off to an individual product page or something.

Thanks.