Dovecot: Geteilte Postfächer (Shared Mailboxes) funktionieren nicht

[hotline at link-m.de]

Hallo,

aktuell ist dovecot bei uns 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)

Code:
# 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?

Vielen Dank.

Viele Grüße
Hotliner