Horde shared mailboxes

I'm trying to set up one mailbox to be shared with another one using steps described here but for some reason I do not see that option in my mailbox (logged as an admin) even I have set ACL as a true in the IMP configuration.

From dovecot -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
}

Any tips on that?

It came up that for some reason backends.local.php shouldn't be a modified copy of backends.php. Simple - avoid default array notification for config but rather to set it as

<?php
$servers['imap']['disabled'] = true;
$servers['advanced']['disabled'] = false;
$servers['advanced']['secure'] = 'tls';
$servers['advanced']['debug'] = '/tmp/imp_imap.log';
$servers['advanced']['debug_raw'] = true;