If it can helps anyone, I put that here :
I just migrate from 11 to 12 and get the message “This directory is unavailable” (in french : “Ce dossier n’est pas disponible”) when displaying the root directory of the File App.
Error was caused by the File Sharing App (disable the app solved the problem immediately), so I tried to find the reason of the failure ant it appears that all of my shares was stored in the oc_share table with uid_initiator=NULL.
I fix that with this short SQL command :
update oc_cache set uid_initiator=uid_owner
assuming that all shares are initiated by the owner of the file.
Hope this helps…