HKLM\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters,EnablePlainTextPasswordAllowInsecureGuestAuthEnableSecuritySignatureRequireSecuritySignature
HKLM\SYSTEM\CurrentControlSet\Services\LanmanServer\Parameters,EnableAuthenticateUserSharingEnableSecuritySignatureRequireSecuritySignature
Windows 10 will try to negotiate SMB3_11, which Samba4 doesn't yet support except in the current 4.3 release candidate.I suspect for now disabling SMB2/3 on the Windows 10 client is your best, if not ideal, option.
[I haven't been doing Save & Secure updates. When massive changes, things tend to go bad for me. Have been doing just certain system plugins lately like .NET, VC++, one at a time. ]
I just did Safe & Secure on fresh 12/24/15 package. Network = v38 before and after.ChrisR said latest it is V39. I was looking for Advanced button ...
I was wrong on attachment, it seems I modified the previous post with the Network plugin attached now, not VC++runtimes.
other options are with your setting by default, can you confirm that it matches ?