Fathi Arfaoui on "Unable to upload media to media library"

You need to set the permission in the "uploads" file, it happened to me two weeks ago when the hosting provider changed their server settings and permissions for some security.
Verify first that you can change permissions to 777 for the actual month "01" in the year file, sometimes you see that the permissions have been changed but when you refresh the page , it come back to the previous settings.
So,change those permissions for the "uploads" file. if you still have the issue, you need to contact again your web hosting, they will change permissions for your uploads file.