What does Cannot generate thumbnail mean?

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Tags

1. #device
2. #loading
3. #sometimes
4. #thumbnail
5. #thumbnails
6. #gallery
7. #however
8. #search
9. #android
10. #reads
11. #speed
12. #which
13. #phone
14. #folders
15. #storage
16. #album
17. #caches
18. #system
19. #photo