Additional note: The files are uploaders in our server, I can see them via FTP, however they don’t show in the front-end!
Note that we run the last version of the plugin, and that the bug presents itself also when we de-activate all plugins / custom design.
So, it doesn’t look like a conflict with another plugin.
Finally, the bug disappears if I empty the user folder inside /user_uploads/
It looks like a problem among file names!