[Samba] Access denied: not in valid users, not in valid format

[Chris]

Dear All,

when trying to open a Samba 3 share, access is denied.

This is the share:

[public]
   comment = Public-Dir
   path = /home/groups/public
   force create mode = 0664
   valid users = @users
   write list = @users

This is the logfile:

[2014/05/23 11:12:02.992788,  3]
../libcli/security/dom_sid.c:208(dom_sid_parse_endp)
  string_to_sid: SID @users is not in a valid format

Another error message:

[2014/04/08 15:09:36.805565, 10] smbd/share_access.c:219(user_ok_token)
  User xy not in 'valid users'
[2014/04/08 15:09:36.805620,  2]
smbd/service.c:627(create_connection_session_info)
  user 'xy' (from session setup) not permitted to access this share
(public)
[2014/04/08 15:09:36.805701,  1] smbd/service.c:805(make_connection_snum)
  create_connection_session_info failed: NT_STATUS_ACCESS_DENIED

Linux users can access the public-directory, but no Windows users. It is
a standalone Samba Server, no Domain Member. All users are linux users,
there's no LDAP.

Thank you in advance.

- Chris