samba - Jun 2017 - unix charset = ISO-8859-15 and samba-4.6.* issue

hi all,

I am experiencing an issue where a samba server is version 4.6.* and in
smb.conf appears:

unix charset = ISO-8859-15

if a client is version 4.5, no problem.

If a client is version 4.6, smbclient fails:

  /opt/samba/bin/smbclient -Uuser%pass -L localhost -d10
[...]
smb_signing_sign_pdu: sent SMB signature of
[0000] 42 53 52 53 50 59 4C 20                             BSRSPYL
convert_string_talloc: Conversion error: Illegal multibyte sequence()
SPNEGO login failed: Undetermined error
session setup failed: NT_STATUS_UNSUCCESSFUL

On server the log reads:

[...]
 NativeOS=[Unix] NativeLanMan=[Samba] PrimaryDomain=[]
[2017/06/09 11:43:01.509339,  3]
../lib/ldb-samba/ldb_wrap.c:325(ldb_wrap_connect)
  ldb_wrap open of secrets.ldb
[2017/06/09 11:43:01.512271,  3]
../auth/ntlmssp/ntlmssp_util.c:69(debug_ntlmssp_flags)
  Got NTLMSSP neg_flags=0x62088215
[2017/06/09 11:43:01.513515,  3]
../source3/smbd/server_exit.c:246(exit_server_common)
  Server exit (failed to receive smb request)
[2017/06/09 11:43:01.519089,  3]
../source3/lib/util_procid.c:54(pid_to_procid)
  pid_to_procid: messaging_dgm_get_unique failed: File or directory not
found

If I remove the:

unix charset = ISO-8859-15

smbclient works again.

But unfortunately I have a slew of legacy data with filenames encoded as
ISO-8859-15 and swapping to a UTF-8 is not feasible,

How can I solve it?

thank you,

Francesco