Comment 4 for bug 2003595

As Athos noted this error is present in the logs:

  ERROR: The 'security=ADS' mode requires 'realm' parameter to be set!

however, according to smb.conf(5) the default security mode is user (security = user), ADS has to be set explicitly (security = ads). In the conf file you attached [1] I can't see this, so there is likely some other config file/option somewhere else. Could you please check?

This looks like a local configuration issue to me, and not a bug in Ubuntu. The right channel for this kins of questions are listed here:

  https://ubuntu.com/community/support

[1] https://launchpadlibrarian.net/647931319/smb.conf