Had an error on the “cart” page when a woocommerce plugin was activated. After looking into it, it seems like the WooCommerce template cart.php passes 3 variables to the woocommerce_cart_item_quantity filter.

Your cart template needs to pass 3, instead of 2:

so instead of :
echo apply_filters( ‘woocommerce_cart_item_quantity’, $product_quantity, $cart_item_key );

need
echo apply_filters( ‘woocommerce_cart_item_quantity’, $product_quantity, $cart_item_key, $cart_item );