Webmail da Horda não reconhece ACLs Dovecot

Estou tentando configurar uma caixa de correio para ser compartilhada com outra usando as etapas descritas aqui, mas por algum motivo não vejo essa opção na minha caixa de correio (registrada como administrador), mesmo que configurei ACL como verdadeira na configuração do IMP .

De pombal -n:

auth_debug = yes
auth_debug_passwords = yes
auth_master_user_separator = *
auth_mechanisms = plain login
auth_verbose = yes
auth_verbose_passwords = yes
dict {
  acl = mysql:/usr/etc/dovecot/dovecot-dict-sql.conf.ext
}
disable_plaintext_auth = no
lda_mailbox_autosubscribe = yes
log_path = /var/log/dovecot.log
mail_home = mdbox:/mnt/homedirs/%2Mu/%2.2Mu/%u
mail_location = mdbox:/mnt/mailboxes/%2Mu/%2.2Mu/%u
mail_plugins = acl
mail_shared_explicit_inbox = yes
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy include variables body enotify environment mailbox date index ihave duplicate mime foreverypart extracttext vnd.dovecot.pipe vnd.dovecot.filter vnd.dovecot.execute
namespace {
  list = children
  location = mdbox:/mnt/mailboxes/%%2Mu/%%2.2Mu/%%u
  prefix = shared/%%u/
  separator = /
  subscriptions = yes
  type = shared
}
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    auto = subscribe
    special_use = \Drafts
  }
  mailbox INBOX {
    auto = subscribe
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = \Trash
  }
  prefix =
  separator = /
  subscriptions = yes
}
passdb {
  args = /usr/etc/dovecot/mastership-sql.conf
  driver = sql
  master = yes
  pass = yes
}
passdb {
  args = /usr/etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
plugin {
  acl = vfile
  acl_defaults_from_inbox = yes
  acl_shared_dict = proxy::acl
  sieve_execute_socket_dir = sieve-execute
  sieve_extensions = +vnd.dovecot.execute +vnd.dovecot.filter +vnd.dovecot.pipe
  sieve_filter_socket_dir = sieve-filter
  sieve_pipe_socket_dir = sieve-pipe
  sieve_plugins = sieve_extprograms
}
service dict {
  unix_listener dict {
    user = dovecot
  }
}
service imap-postlogin {
  executable = script-login /usr/etc/dovecot/imappostlogin
  user = $default_internal_user
}
service imap {
  executable = imap imap-postlogin
}
ssl = no
ssl_cert = </etc/pki/tls/certs/hostname.bundle
userdb {
  args = uid=dovecot gid=dovecot home=/mnt/mailboxes/%%2Mu/%%2.2Mu/%%u
  driver = static
}
protocol lmtp {
  mail_plugins = acl sieve
}
protocol lda {
  mail_plugins = acl sieve
}
protocol imap {
  mail_plugins = acl imap_acl
}

Alguma dica sobre isso?