Collaboration Folders and Documents are setup with Access 'All Participants'. However, no all the Project Participants can access the documents. Why?.
That behavior is not as expected. For some reason the participants were not added to the cmn_sec_assgnd_obj_perm TABLE.
WORKAROUND: The Collaboration needs to remove the participants that can not see the documents and re-added them as participants, that way the necessary rights will be added.