After enabling also CERT-auth on server, Ubuntu 18 clients fail to do PSK authentication.
Looks like wrong TLS-handshake happens. Ubuntu 18 zabbix_agent-package is also linked with very old openssl.
- zabbix_agentd --version | egrep "daemon|SSL"
zabbix_agentd (daemon) (Zabbix) 4.0.22
This product includes software developed by the OpenSSL Project
for use in the OpenSSL Toolkit (http://www.openssl.org/).
Compiled with OpenSSL 1.1.0g 2 Nov 2017
Running with OpenSSL 1.1.1 11 Sep 2018