Dovecot: Geteilte Postfächer (Shared Mailboxes) funktionieren nicht

[Cesar]

Hallo,

aktuell ist dovecot nicht in der Lage, geteilte Postfächer einzurichten/zu nutzen, obwohl in der Version und Konfiguration das gehen sollte.
Besonderheit: es wird gegen das AD über LDAP authentifiziert, dieser Teil funktioniert schon seit Jahren.

OS: Linux 5.4.0-88-generic x86_64 Ubuntu 20.04.3 LTS

doveconf -n: (nicht Standard-Konfig)

# 2.3.16 (7e2e900c1a): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.16 (09c29328)
# OS: Linux 5.4.0-88-generic x86_64 Ubuntu 20.04.3 LTS
# Hostname: <hostname>
auth_master_user_separator = *
auth_mechanisms = plain login
auth_worker_max_count = 60
default_client_limit = 1500
default_process_limit = 1500
default_vsz_limit = 384 M
disable_plaintext_auth = no
first_valid_uid = 100
imap_id_log = *
mail_location = maildir:~/Maildir:UTF-8
mail_plugins = acl
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 imapflags notify
namespace {
  list = children
  location = maildir:%%h/Maildir:INDEX=~/Maildir/shared/%%u/
  prefix = shared.%%u.
  separator = .
  subscriptions = no
  type = shared
}
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix =
}
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
passdb {
  args = /etc/dovecot/passwd.masterusers
  driver = passwd-file
  master = yes
  pass = yes
}
passdb {
  args = username_format=%u /etc/dovecot/passwd.users
  driver = passwd-file
}
plugin {
  acl = vfile:/etc/dovecot/dovecot-acl
  acl_globals_only = yes
  sieve = ~/.dovecot.sieve
  sieve_dir = ~/sieve
  sieve_extensions = +notify +imapflags
}
protocols = imap pop3 sieve
service anvil {
  unix_listener anvil-auth-penalty {
    mode = 00
  }
}
service auth {
  unix_listener /var/spool/postfix/private/dovecot-auth {
    group = postfix
    mode = 0660
    user = postfix
  }
  unix_listener auth-userdb {
    group = root
    mode = 0600
    user = vmail
  }
}
service imap-login {
  process_limit = 1024
}
service imap {
  process_limit = 1024
  vsz_limit = 2 G
}
service managesieve-login {
  inet_listener sieve {
    port = 4190
  }
  inet_listener sieve_deprecated {
    port = 2000
  }
}
service stats {
  unix_listener stats-reader {
    group = postfix
    mode = 0660
    user = vmail
  }
  unix_listener stats-writer {
    group = postfix
    mode = 0660
    user = vmail
  }
}
ssl_ca = </etc/ssl/daemon/<hostname>.bundle
ssl_cert = </etc/dovecot/dovecot.pem
ssl_cipher_list = ALL:!LOW:!SSLv2:ALL:!aNULL:!ADH:!eNULL:!EXP:RC4+RSA:+HIGH:+MEDIUM
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
userdb {
  args = uid=vmail gid=nogroup home=/var/vmail/%Ld/%Ln allow_all_users=yes
  driver = static
}
userdb {
  args = username_format=%u /etc/dovecot/passwd.users
  driver = passwd-file
}
protocol imap {
  imap_client_workarounds = delay-newmail
  mail_max_userip_connections = 100
  mail_plugins = acl acl imap_acl
}
protocol pop3 {
  mail_max_userip_connections = 20
  pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
}
protocol lda {
  deliver_log_format = msgid=%m: %$
  mail_plugins = sieve sieve
  quota_full_tempfail = yes
  rejection_reason = Your message to <%t> was automatically rejected:%n%r
}

<hostname> ist nur ein Platzhalter für einen FQDN, natürlich ohne die spitzen Klammern.

Geteilte Postfächer sind heute eine häufige Anforderung, in MS Exchange usw. geht das sehr einfach, aber mit dovecot ist das bei nicht Standardkonfiguration (Separator = . ) anscheinend nicht trivial.

Hat jemand hier eine Lösung oder Tipps die zum Ziel führen könnten?