Can somebody provide an example smb.conf for a NT domain controller?

After I updated my domain server from Ubuntu 18.04 to 20.04 the NT domain no longer worked. Domain logons on the Windows 10 computers fail. (Ubuntu 18.04 is no longer supported).

So I set up a new (virtual) machine with Ubuntu 20.04 and a new domain name in order to trouble shoot this problem. I get as far as "successfully" adding a Windows 10 computer to the domain ("Successfully" meaning "without any error messages"), but when I reboot that Win10 box, domain logons fail with the error: "We can't sign you in ... because your domain isn't available."

If I log on with a local account, I can access the shares on the domain controller using a domain account (connect using different credentials).

This is my smb.conf:

# Global parameters
[global]
    add machine script = /usr/sbin/useradd -N -g computers -c "Computer %u" -d /dev/null -s /bin/false %u
    dns proxy = No
    domain logons = Yes
    domain master = Yes
    log file = /var/log/samba/log.%m
    logon drive = z:
    logon home = \\dcubuntu18\netlogon
    logon path = \\%N\profiles\%U
    logon script = logon.cmd
    map to guest = Bad User
    max log size = 1000
    name resolve order = lmhosts host wins bcast
    obey pam restrictions = Yes
    os level = 65
    pam password change = Yes
    panic action = /usr/share/samba/panic-action %d
    passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
    passwd program = /usr/bin/passwd %u
    security = USER
    server role = classic primary domain controller
    server string = %h server
    time server = Yes
    unix extensions = No
    unix password sync = Yes
    usershare allow guests = Yes
    wins support = Yes
    workgroup = TWMDOMAIN
    idmap config * : backend = tdb
    admin users = @DomainAdmins


[homes]
    browseable = No
    comment = Home Directories
    valid users = %S


[netlogon]
    comment = Network Logon Service
    guest ok = Yes
    path = /home/netlogon


[printers]
    browseable = No
    comment = All Printers
    create mask = 0700
    path = /var/spool/samba
    printable = Yes


[print$]
    comment = Printer Drivers
    path = /var/lib/samba/printers


[etc_samba]
    browseable = No
    path = /etc/samba

(The etc_samba share is there for allowing me to use windows tools to view the smb.conf file. This is of course not a production system.)

Could somebody please share with me a working smb.conf file and possibly other information on how to set up a new Samba server with Ubuntu 20.04 (Samba Version 4.7.6-Ubuntu) or later and a NT domain (not an AD domain)?