Missing essence picture thumbnails

[afdev82]

I'm using S3 to store the images on latest Alchemy v5.
I just copied all my local pictures to S3 and I realized that in the admin interface I was missing the essence picture thumbnails (I get some 403 errors from URL like http://[bucket_url]/production/alchemy/pictures/pictures/295/8ff893a5a2ff20a47352fdffb303165544cec60f/Bildgruppe_30_2x.png)
In the frontend I have the pictures and in the picture library too (thumbs & pictures).
If I run the provided rake tasks, it logs URLs like this one:

http://[bucket url]/production/alchemy/pictures/pictures/289/39770289501ebaf8e3b04c0bd72b9813f1d6bcdf/MW_ascent_model_mirror_4_2x.png

If I go to this URL I get Access Denied, the picture is not there If I check directly in Amazon S3.

It doesn't happen with all essence picture thumbs, maybe is it an issue with the filenames?
Eg. MW [email protected] is the filename, but in the request and in the thumb task generation task I get MW_ascent_model_mirror_4_2x.png
Also in the admin interface I get no thumbnails after choosing the image. With other images on the same essence, it works.
For the same image I have the thumbnail in the picture gallery.