Some products disappear from cart when logging out
I have a strange situation a customer just found.
We have 11 main categories with about 2500 products total. When any products from 1 of these main categories, called "Organic", is placed in a customer's cart, they log out and log back in, it's gone. The products in all other main categories correctly remain saved in the customer's cart.
I've tried to figure out the difference between this one category from the others. It is the only one starting with an ě (scandinavian letter), so I changed the name so it has no special characters. It still happens.
I tried linking a product from another category to also exist in this troubled category. That product, when placed in the cart from the problem cat, remains saved correctly when logging out.
I tried linking a product FROM this problem cat into a category that works okay. When put in the customer's cart from one of the normally-working categories, this product disappears when logging out.
So, it seems there's something different about this one category and I can't find it. It is the last cat I added (a long time ago) but otherwise I don't know.
Can someone please point me in the right direction? I'm stuck.
Currently using ZEN CART v. 1.3.6