[Dovecot] Master user sees empty folder list

[interfaSys sàrl]

Hello,

I'm having a "path" problem with master users. When logged on as a
master user, I can't see the user's folders.

I'm trying to use system users as master users, so I'm using something
like this:

# Master password
auth_master_user_separator = *
passdb {
 driver = passwd
 master = yes
 pass = yes
}

# System users
passdb {
 driver = passwd
}

# Virtualusers users
passdb {
 driver = passwd-file
 args = username_format=%n /etc/virtual/%d/passwd
}
mail_location = mdbox:~/mdbox

System and virtual users see their folders fine, including the shared
namespaces, but as a master user, I see an empty mailbox.

I've turned on debugging and watched things like root= and fs= and
they're normal.
Permissions should be OK since the system user has the same uid:gid as
the virtual user.

What could it be?

Cheers,

Olivier