ok, we have changed it and it worked!
For some strange reason, now the user is unable to upload files. He sees the files, but can't upload them. He is not an admin, so it is a bit strange to me that he was able to upload them in the first place. Do you have any idea of what's going on:
https://faenet.org/community/pages/34-folleterre/46-files
you can log in with a test user: itmrj80@gmail.com password: qwer1234