Zabbix server is considering that a client is unavailable even when communication works very well from server to client.

Server logs:
9697:20131203:143619.729 In get_value_agent() host:'comprasB' addr:'192.168.3.188' key:'agent.ping'
9697:20131203:143621.737 Item [comprasB:agent.ping] error: Received empty response from Zabbix Agent at [192.168.3.188]. Assuming that agent dropped connection because of access permissions.
9697:20131203:143621.738 query [txnlev:1] [update hosts set error='Received empty response from Zabbix Agent at [192.168.3.188]. Assuming that agent dropped connection because of access permissio',disable_until=1386088641 where hostid=10071]

Client logs:
10851:20131203:143610.902 In update_cpustats()
10851:20131203:143610.902 End of update_cpustats()
10851:20131203:143610.902 collector [idle 1 sec]
10853:20131203:143611.349 listener #2 [processing request]
10851:20131203:143611.902 collector [processing data]
10851:20131203:143611.902 In update_cpustats()
10851:20131203:143611.902 End of update_cpustats()
10851:20131203:143611.902 collector [idle 1 sec]
10853:20131203:143612.350 listener #2 [waiting for connection]
10851:20131203:143612.903 collector [processing data]

1. cat /etc/zabbix/zabbix-agentd.conf
   Server=zabbix.mydomain.com
   Hostname=comprasB
   LogFile=/var/log/zabbix/zabbix_agentd.log
   PidFile=/tmp/zabbix_agentd.pid
   StartAgents=5
   DebugLevel=4
   Timeout=3
   RefreshActiveChecks=60
   EnableRemoteCommands=1
   LogRemoteCommands=1
   UnsafeUserParameters=1

zabbix_get tests:

1. zabbix_get -s 192.168.3.188 -k 'agent.version'
   2.2.0
2. zabbix_get -s 192.168.3.188 -k 'agent.ping'
   1