[Samba] smbclient //server/netlogon -k -c 'ls' fails with "NT_STATUS_LOGON_FAILURE"

[Arcadie Cracan]

Dear all,

After recently upgrading my server from Debian Jessie to Debian Stretch, 
Windows 7 clients fail to join (or even authenticate) with the samba ad server.

I have redone the basic checks on the samba server:
smbclient //localhost/netlogon -U Administrator -c 'ls'  -- works
kinit administrator at INTERNAL.DOMAIN.COM -- works
klist -- works
smbclient //server/netlogon -k -c 'ls' -- fails

Detailed output:
smbclient //lotus.intra.dam-application.ro/netlogon -k -c 'ls' -d9

Any advices for troubleshooting this issue?

Thank you for your help!

   Arcadie Cracan