Comment 13 for bug 1761737

Ok, so to summarize:
- sssd is providing user and groups from AD (via /etc/nsswitch.conf)
- realmd was used to join the machine to AD for the above
- local user authentication is done via pam_sss and using kerberos. Shell users get a ticket upon login
- samba is not using winbind

I have a feeling samba is missing it's account with the AD server. I don't know if the sssd join works for samba's "security = ADS", I have never tested that. I always used net ads join. Is this how you configured the non-18.04 samba member servers? With just sssd, no "net ads join"?

The crash also seems to indicate that the "secrets" bit of "secrets and keytab" is returning a null pointer to the code, so maybe samba isn't finding the secret.

Do you have a populated /etc/krb5.keytab?

Can you try these commands:
net ads testjoin -k
net ads status -k

After having acquired a kerberos ticket most likely (for -k to work).