Steps to reproduce:
1. Login as admin
2. Go to "Page" -> "site"
3. Start creating a new Landing Page
4. Set required fields: Title and Description ("TestLP")
5. Add two Code Blocks with content: "Block1" and "Block2"
6. Save Landing Page Draft
7. Exit Landing Page editor by switching to view mode
8. Go to Admin -> Roles -> Anonymous
9. Delete the content/read policy from Anonymous role
10. Logout from all siteaccesses (I recommend closing the browser and opening a new private window)
11. Login as admin
12. From the Dashboard click the edit button next to "TestLP" in "Drafts" section"
Page is rendered correctly
Page Builder does not load.
I've noticed that I was not authenticated as admin user in EzSystems\EzPlatformPageFieldTypeBundle\Controller\BlockController::renderAction(), but a Anonymous instead.