Really strange WSOD after submitting a form/add to cart
I had a similar issue a year and a half ago. But in my case, it was Drupal 7 with Ubercart. Drupal had the same behaviour that you has described in your post. 

If my memory serves my right, the problem was that references to the products  when the cart is executed. They are stored in 2 different ways:


If you are a authenticated user (you have an account on a drupal site), the references to the products added to the cart are stored in database. Concretely, in a table called *uc_cart_products*. If you log out, the products remain safely in database until the user log in again. So, the user doesn't miss any information.
If you are an anonymous user (not authenticated), the references are stored in database as in step 1, but you have a radomly generated token that is stored in Cookies to identify you . The cookie has a limited time of life, so finally, the cookie will be deleted.


When the cart is built, Ubercart take that reference from cookies if it's needed and load the products. But, i had deleted some products manually from database ( i know, it's a bad deccision ), so when the system try loading a product that hadn't exist, the system failed and appeared WSOD.

To solve this problem:


I deleted manualy all lines in *uc_cart_products* database table. If you have lots of records, only delele your products. Each record has an user id or a token assigned to identify who is the user. 
Log out and delete the cookies