# /etc/zabbix/zabbix_agentd.conf 
PidFile=/var/run/zabbix/zabbix_agentd.pid
LogFile=/var/log/zabbix/zabbix_agentd.log
LogFileSize=0
Server=<IP>
ServerActive=<IP>
Hostname=psk-host
Include=/etc/zabbix/zabbix_agentd.d/*.conf
TLSConnect=psk
TLSAccept=psk
TLSPSKIdentity=PSK-123
TLSPSKFile=/etc/zabbix/zabbix_agentd.psk

# command on Zabbix "Client" Ubuntu 18.04
zabbix_sender -c /etc/zabbix/zabbix_agentd.conf -k ein.test -o 12 -vv
zabbix_sender [21733]: DEBUG: In zbx_tls_init_child()
zabbix_sender [21733]: DEBUG: OpenSSL library (version OpenSSL 1.1.1  11 Sep 2018) initialized
zabbix_sender [21733]: DEBUG: zbx_tls_init_child() loaded PSK identity "PSK-123"
zabbix_sender [21733]: DEBUG: zbx_tls_init_child() loaded PSK from file "/etc/zabbix/zabbix_agentd.psk"
zabbix_sender [21733]: DEBUG: zbx_tls_init_child() PSK ciphersuites: TLS_AES_256_GCM_SHA384 TLS_CHACHA20_POLY1305_SHA256 TLS_AES_128_GCM_SHA256 ECDHE-PSK-AES128-CBC-SHA256 ECDHE-PSK-AES128-CBC-SHA PSK-AES128-GCM-SHA256 PSK-AES128-CCM8 PSK-AES128-CCM PSK-AES128-CBC-SHA256 PSK-AES128-CBC-SHA
zabbix_sender [21733]: DEBUG: End of zbx_tls_init_child()
zabbix_sender [21734]: DEBUG: In zbx_tls_connect(): psk_identity:"PSK-123"
zabbix_sender [21734]: DEBUG: zbx_psk_client_cb() requested PSK identity "PSK-123"
zabbix_sender [21734]: DEBUG: End of zbx_tls_connect():SUCCEED (established TLSv1.3 TLS_AES_256_GCM_SHA384)
zabbix_sender [21734]: Warning: SSL_shutdown() with <IP> set result code to 1: file ../ssl/ssl_lib.c line 2072: error:140E0197:SSL routines:SSL_shutdown:shutdown while in init
zabbix_sender [21734]: DEBUG: send value error: TLS read set result code to 1: file ../ssl/record/rec_layer_s3.c line 1528: error:1409445C:SSL routines:ssl3_read_bytes:tlsv13 alert certificate required: SSL alert number 116: TLS read fatal alert "unknown"
Sending failed.

# Errors in Zabbix Agent Log with Active checks
 20476:20200116:153833.766 SSL_shutdown() with <SERVER-IP> set result code to 1: file ../ssl/ssl_lib.c line 2072: error:140E0197:SSL routines:SSL_shutdown:shutdown while in init
 20476:20200116:153833.766 active check configuration update from [<SERVER-IP>:10051] started to fail (TLS read set result code to 1: file ../ssl/record/rec_layer_s3.c line 1528: error:1409445C:SSL routines:ssl3_read_bytes:tlsv13 alert certificate required: SSL alert number 116: TLS read fatal alert "unknown")
...
 20476:20200116:153933.787 SSL_shutdown() with <SERVER-IP> set result code to 1: file ../ssl/ssl_lib.c line 2072: error:140E0197:SSL routines:SSL_shutdown:shutdown while in init