[Samba] samba-tool cannot add or remove group members

[Sébastien QUESSON]

Hi

samba-tool group addmembers stage user
ERROR(exception): Failed to add members "user" to group "stage" - Unable to find "user". Operation cancelled.
  File "/usr/lib/python2.7/dist-packages/samba/netcmd/group.py", line 227, in run
    add_members_operation=True)
  File "/usr/lib/python2.7/dist-packages/samba/samdb.py", line 274, in add_remove_group_members
    raise Exception('Unable to find "%s". Operation cancelled.' % member)

net rpc group addmem stage user -UAdministrator
>ok, user added

samba-tool group removemembers stage user
>error

net rpc group delmem stage user -UAdministrator
> ok user deleted

I remember that I provisionned my domain few weeks ago with samba-tool without problem!
This command does not work anymore for another user, or for other groups...

What's the problem? I'm on debian 9, samba 4.5.8
I read some similar problem on the list https://www.spinics.net/lists/samba/msg142240.html, but I did not help me

Best regards, 

Sébastien QUESSON