When Content A has a relation pointing to the Content B and active User tryies to view/edit Content A without permissions to read Content B 500 exception will be thrown:

HTTP 500 Internal Server Error
An exception has been thrown during the rendering of a template ("User does not have access to 'read' 'content' with: contentId '58'") in "@admin/fieldtypes/preview/content_fields.html.twig".

Steps to reproduce
1. Clean installation of eZ Platform 2.5.6
2. Log into the backoffice as admin.
3. Create a new Content Type Content With Relation containing ezrelation field.
4. In Home create and publish Folder Folder A.
5. In Home create and publish Folder Folder B.
6. In Folder B create and publish Article Test Article.
7. In Folder A create and publish Content With Relation Test with relation to Test Article.
8. Edit Editor Role:

9. Create Editor user in Editors Group.
10. Log in to the backoffice as Editor.
11. In Content Structure go to Test

Result
In dev:

HTTP 500 Internal Server Error
An exception has been thrown during the rendering of a template ("User does not have access to 'read' 'content' with: contentId '58'") in "@admin/fieldtypes/preview/content_fields.html.twig".

In prod:
Partially rendered page with not working buttons.

Expected Result
Content view of Test opens normally.