"This view has been automatically updated to fix missing relationships" - fields removed from a working view for no reason
EDIT: It turned out after all that this behaviour was related to the "Administration Language" module https://drupal.org/project/admin_language‎.
Disabling it made the problem disappear for good.
It seems as its is related to access permissions.
A similar problem was encountered in creating product views in drupal commerce. If you want to relate to the fields of the referenced product, you have to grant the View permisssion to all products (non-node enitities of DC). Otherwise the users without permission to view the product enitities will see an empty view (see Views results empty for unprivileged user when using Relationship: Content: Referenced Product). What makes it especially difficult to track that the problem does show up only when one uses Relationship &amp; "Fields" display. A similar view with content / teaser etc renders the same content without problems.

Now, I wanted to add an image of the product to the cart view. I referenced the Image field of the product in the cart ((Line Item) Commerce Line item: Product), and put the image field from there. 

I understand that the permissions problem prevent the view from functioning properly. It seems to be that it's a bug that
a) the view itself functions well
b) that the problem appears in the Views UI
c) that the error message is not informative.     

It seems to me that this behavior is just a side effect of a number factors in how the Views work.