Hi,
It's a crazy thing.
I finally solved it. I reinstalled the latest nis and samba schemas (I already did it when upgrading to version 6), I deleted the Posix group where I was linked, created it again, associated it to all the users concerned. Stop/start zimbra, and now it works.
Don't ask me to explain why but it works
Anyway MANY MANY THANKS for your help !
You give me a very interesting hint with the LDAP browser that may be very useful for future Zimbra administration.