Hi

I have appended the important bits of my server configuration to the end of this email
I am trying to get "Mailbox sharing between users" to work.

For starters it is very hard to follow the doc only
https://doc.dovecot.org/configuration_manual/shared_mailboxes/shared_mailboxes/
without an actual example. I tried to use this (sorry in German)
https://dokuwiki.tachtler.net/doku.php?id=tachtler:dovecot_shares
and this got me a little further down the track.

I followed the instructions required for the dovecot config files.
Restarted dovecot, not errors displayed in the logs.

I have got access to two user accounts, say USER1 and USER2.
I executed this lines

doveadm acl set -u USER1 INBOX user=USER2 admin create delete expunge insert lookup post read write write-deleted write-seen
doveadm acl set -u USER2 INBOX user=USER1 admin create delete expunge insert lookup post read write write-deleted write-seen

and created (just to be sure) two folders for each user

/home/USER1/Mail/shared/USER1
/home/USER1/Mail/shared/USER2
/home/USER2/Mail/shared/USER1
/home/USER2/Mail/shared/USER2

The tried to create a mailboxes like so

/home/USER1/Mail/shared/USER1/testa
/home/USER1/Mail/shared/USER2/testb
/home/USER2/Mail/shared/USER1/testa
/home/USER2/Mail/shared/USER2/testb

They are not created and no error is displayed - even looking at the dovecot logs.
What I see in the folders for BOTH users though (for every folder)

dovecot.list.index.log

On the command line I can see the .imap files and do a 'ls -alR' in the folders, but no created mbox.

I can definitely see the folders, but I cannot create a mbox file.

What am I doing wrong?
Help please.

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Here are the important configuration items

# 2.3.16 (7e2e900c1a): /etc/dovecot/dovecot.conf
# OS: Linux 4.18.0-425.13.1.el8_7.x86_64 x86_64 AlmaLinux release 8.7 (Stone Smilodon) xfs
namespace shared {
list = children
location = maildir:~/Mail/:INDEX=~/Mail/shared/%%u:CONTROL=~/Mail/shared/
prefix = shared/%%u/
separator = /
subscriptions = yes
type = shared
}
mail_location = mbox:/home/%u/Mail/:INBOX=/var/mail/%u
mail_plugins = acl
passdb {
driver = pam
}
plugin {
acl = vfile
acl_shared_dict = file:/var/lib/dovecot/db/shared-mailboxes.db
}
service auth {
unix_listener auth-userdb {
    group = dovecot
    mode = 0777
    user = dovecot
}
}
protocol imap {
mail_plugins = " acl imap_acl"
}