dovecot - Sep 2015 - File permissions: system groups for do not work for group/shared mailboxes

Hello!
 
I am struggling to set up a group mailbox for users of a certain system group. I
want to grant access to the mailbox on a filesystem mode by using the system
groups.
The problem, the set up works fine and when I use setfacl and grant permissions
too each user manually, it works fine. However, I want to use a system
user's group membership in order to grant access to these mailboxes insted
of manually granting for each user.
 
dovecot version: 2.2.9
 
This is the config part:
namespace {
  location = maildir:/data/mail/public:LAYOUT=fs
  prefix = GROUP-MAILBOXES/
  separator = /
  subscriptions = no
  type = public
}
 
...
passdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
...
userdb {
  args = /etc/dovecot/dovecot-ldap.conf.ext
  driver = ldap
}
 
filesystem permissions (getfacl):
# file: GroupboxA/
# owner: root
# group: root
user::rwx
user:user1:rwx
group::rwx
mask::rwx
other::---
default:user::rwx
default:user:user1:rwx
default:group::rwx
default:mask::rwx
default:other::---
 
# file: GroupboxB/
# owner: root
# group: root
user::rwx
group::rwx
group:group1:rwx
mask::rwx
other::---
default:user::rwx
default:group::rwx
default:group:group1:rwx
default:mask::rwx
default:other::---
 
user1 is in roup1
 
When setting username-based permissions  (GroupboxA), the mailbox is usable and
subsribable; when setting groupname-based permissions (GroupboxB) it is not
subscribable/usable.
 
Thanks for your help in advance and best regards,
Jens