Make sure the web server has the the rights to WRITE to this folder (and all its subfolders):
/files/

E.g. currently the first thumbnail image is expected to be located here:
http://www.becsa.fr/files/cache/6bf8239682888435289e3fb298dd393c_f3...

But the server tells the file is not there so I'd assume concrete5 does not have the write permissions to that folder.

Hello,
All your thumbs are generating 404 error, meaning they are nowhere to be found.
According to what I see, the system is looking for them in the cache and can't find them. After flushing the cache they should definitely be generated again.

Something that might help is open the editing the block and saving. That will generate a new block ID and might solve the problem.

Also have you tried to contact the flash gallery developer and ask for support?

I would agree with Mainio except that the full size images are present and my guess is they should also be put in the cache.

But frankly his answer still makes more sense than mine :)

The big images do not come from cache, they are straight references to the relative paths of the files, e.g. the first image:
http://www.becsa.fr/files/4813/6248/0024/froid_1.JPG...

This can happen e.g. if the site has been moved because you'd have successfully already uploaded the files in the development environment and not in the live site.

hi,

i've check the write permission and it's ok.
i've also tried to edit the block then save, but no changes.

i should ask directly to the team as you said.