It's not possible to generate preview for Embed, Video and Gallery blocks when selecting Folders outside the Home tree.

Steps to reproduce:
1. Login as admin
2. Create new Landing Page
3. Add one of the following blocks: Video, Embed, Gallery
4. Click "Folder" and select "Media"
5. Confirm selection
6. Submit block

Expected:
Block preview is generated

Actual:
Instead of preview "There was an error during rendering of the block" is displayed.

The following is visible in logs:
app.WARNING: gallery block preview rendering error: User does not have access to 'read' 'content' with: contentId '41'' [] []
(even though I'm admin with all rights).

Note:
It is possible to embed "Home" or Folders in that tree.