Setup a SASL password authentication as described here:
http://www.postfix.org/SASL_README.html

In general:

  /etc/sasl2/smtpd.conf
    pwcheck_method: auxprop
    auxprop_plugin: sasldb
    mech_list: plain login DIGEST-MD5

  /etc/postfix/main.cf:
    smtpd_sasl_path = smtpd

  Add user:
    saslpasswd2 -c -u example.com username

  Check for user:
    sasldblistusers2

  Restart/reconfigure postfix:
    systemctl restart postfix

  Configure some client (like thunderbird) to communicate with mailer (for
tests don't configure TLS):
    server name: your server
    Port: 25
    Connection security: None
    Authentication method: Password, transmitted insecurely
    Username: as above
    Later give password as above

  Try to send some e-mail from client via above mail server.

The error message occurs in journalctl

Jan 03 18:32:55 server postfix/smtpd[13543]: warning: SASL authentication
failure: Could not open db `/etc/sasldb2
Jan 03 18:32:55 server postfix/smtpd[13543]: warning: SASL authentication
failure: Password verification failed
Jan 03 18:32:55 server postfix/smtpd[13543]: warning:
pXXXXXXXX.dip0.t-ipconnect.de[80.XX.XXX.XX]: SASL PLAIN authentication failed:
generic failure

This worked in openSUSE-15.2 before.

I think it's related to the change of database format: BDB -> GNU dbm
And the fact that postfix is not capable of "GNU dbm": postconf -m | grep -i
dbm