[Samba] permissions problem

[Pascal Legrand]

Hello,
i'm using samba 4.1 on a debian stable, samba act as a NT4 PDC.

i have a permission problem in a samba share.
It's a public share where everybody can read/write and it works fine.

In this share, there is a directory with "specials" permissions
drwxrwx--- 20 root   grp2     4096 nov.  16 14:36 Dossier
For me, these permissions allows someone from "grp2" group to go inside 
this directory

but this user :
uid=30876(user) gid=687(grp1) groupes=687(grp1),30001(grp2)
cant go to this directory. As you can see, "grp2" is not the primary 
group of the user but the secondary group.

It worked before i migrate my server from debian wheezy to jessie and 
then from samba 3.6 to samba 4.1

It would seem that the problem appears when the user goes out of a 
prolonged stand-by mode (not sure of reverso translation)

If i disable winbind it works fine
(for information, i need winbind only if i have linux client ?)


Does someone could help to know from where the problem come ?

-- 
Pascal