Cannot subscribe to Public mailbox

I'm trying to set up a Public mailbox, but when I try to subscribe to it in Thunderbird, nothing happens: it doesn't appear as a folder, and when I go back to the Subscribe window, it is unchecked again.

I use postfix local dlivery, which created a Maildir when a mail arrived for this mailbox: /home/others/sharedmail/Maildir/{cur,new,tmp}

(sharedmail is a system user so that Postfix can deliver the mail; it doesn't have a login shell or a password)

To make sure it is not a simple filesystem permissions problem, I did "chmod -R a+rwX Maildir/"

This is my current config ( I tried to keep it short by leaving only what might be relevant ):

# dovecot -n
# 2.1.7: /etc/dovecot/dovecot.conf
# OS: Linux 3.2.0-4-amd64 x86_64 Debian 7.9
...
mail_location = maildir:~/Maildir
namespace {
   list = yes
   location = maildir:/home/others/sharedmail/Maildir
   prefix = Public.
   separator = .
   subscriptions = no
   type = public
}
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     special_use = \Drafts
   }
   mailbox ...
     ...
   }
   prefix =
   separator = .
}
passdb {
   driver = pam
}
plugin {
   sieve = ~/.dovecot.sieve
   sieve_dir = ~/sieve
}
protocols = " imap"
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0660
     user = postfix
   }
}
userdb {
   driver = passwd
}

Also, I'm not sure about the difference between type=public and type=shared. But anyway, I tried both with the same result.