Documentation about Zabbix appliance for v3.0 contains the following section:

1.2 Firewall

The appliance uses iptables firewall with predefined rules:

• Opened SSH port (22 TCP);
• Opened Zabbix agent (10050 TCP) and Zabbix trapper (10051 TCP) ports;
• Opened HTTP (80 TCP) and HTTPS (443 TCP) ports;
• Opened SNMP trap port (162 UDP);
• Opened outgoing connections to NTP port (53 UDP);
• ICMP pakets limited to 5 packets per second;
• All other incoming connections are dropped.

The same section present also in documentation for v3.2 and in translations on other languages. There is no such section for older versions (2.4, 2.2, etc.).

The problem is that NTP protocol uses UDP port 123; but UDP port 53 is used by the DNS service. I don't know what are the real settings on the appliance, but, probably, both ports should be permitted by firewall for normal functioning.