Steps to reproduce:

On Zabbix-client, I create a psk 

skylab@zabbix-client:~$ sudo sh -c "openssl rand -hex 32 > /etc/zabbix/zabbix_agentd.psk"
skylab@zabbix-client:~$
skylab@zabbix-client:~$ cat /etc/zabbix/zabbix_agentd.psk
b9b82ded68b781b73e83801ec54e0dbd4837b4b6631e5631e58e4145a22000e1

Then I changed the configuration of zabbix

Next I restart zabbix agent

skylab@zabbix-client:/etc/zabbix$ sudo systemctl restart zabbix-agent

and here is zabbix path location

skylab@zabbix-client:/etc/zabbix$ ls -lrt /etc/zabbix/
total 32
drwxr-xr-x 2 root root 4096 Oct 29 2017 zabbix_agentd.conf.d
rw-rr- 1 root root 10295 Dec 28 11:18 zabbix_agentd.conf.orig
rw-rr- 1 root root 65 Dec 28 13:49 zabbix_agentd.psk
rw-rr- 1 root root 10309 Dec 28 14:28 zabbix_agentd.conf

Then I input the value to zabbix front end at Configuration/ Host

Host name: zabbix-client

IP address: 10.10.1.91

Templates: Template OS Linux

Encryption: PSK; PSK identity: PSK 001, PSK: /etc/zabbix/zabbix_agentd.psk

Click Add button and alert appears: 

• Incorrect value for field "tls_psk": an even number of hexadecimal characters is expected.
  Check log server, I found

19682:20181228:141632.514 cannot send list of active checks to "127.0.0.1": host [Zabbix server] not monitored
19682:20181228:141715.503 failed to accept an incoming connection: from 10.10.1.91: TLS handshake set result code to 1: file ../ssl/statem/statem_srvr.c line 2077: error:1419E0DF:SSL routines:tls_process_cke_psk_preamble:psk identity not found: TLS write fatal alert "unknown PSK identity"

But, the PSK was generated correct. So I don't know why? Could you please help me fix it? or give me any advices for this case?

Thank you so much!