I found what the problem was. It seems that using ‘.’ as a separator worked. But somehow folders and domain names with dots got messed up:
maildir++: root=/var/mail/vmail/francisaugusto/francis, index=/var/mail/vmail/med-lo.eu/francis/shared/index/francis@francisaugusto, indexpvt=/var/mail/vmail/med-lo.eu/francis/shared/francis@francisaugusto, control=, inbox=/var/mail/vmail/francisaugusto/francis, alt= Sep 25 19:10:57 imap(francis@med-lo.eu)<58061><skunigB>: Debug: Namespace shared.francis@francisaugusto.: Creating storage despite: Root mail directory doesn't exist: /var/mail/vmail/francisaugusto/francis
You see, my domain is francisaugusto.com http://francisaugusto.com/, but it got truncated.
I changed the separator to ‘/‘ on the Inbox namespace and on the shared one, and voila, it worked.
I can see my shared mailboxes everywhere. But on Apple Mail (both on the Mac and on the iPhone), the shared mailboxes show no message.
Any clue why?