need help error receiving answer on HELO request

[damainbn]

Error Message:
Subject:
PROBLEM: Zabbix agent on Radius is unreachable for 5 minutes

Message:
Trigger: Zabbix agent on Radius is unreachable for 5 minutes
Trigger status: PROBLEM
Trigger severity: Average
Trigger URL:

Item values:

1. Agent ping (Radius:agent.ping): Up (1)
2. *UNKNOWN* (*UNKNOWN*:*UNKNOWN*): *UNKNOWN*
3. *UNKNOWN* (*UNKNOWN*:*UNKNOWN*): *UNKNOWN*

Error:
error receiving answer on HELO request: [4] Interrupted system call



What is this? and how to fix it?
zabbix ver. 2.0.4