Very interesting. I tested this and I also get those same errors. And only when adding the Product Kit from the Catalog - works fine from the Product page, like you said. Interesting also because this is the same error I was seeing when I tried to get my Reorder Button contribution (http://www.ubercart.org/contrib/3734) working for Product Kits. (Reorder button will add everything from a previous order into your current cart.)

It appears that there is some missing information or some missing step that is needed for Product Kits that is only done on the product page, not on the cart page and not with my Reorder button either. I wonder what it is...