Zabbix stopped to collect data on a lot of our servers. The problem didn't occur on all hosts simultaneously. The queue keeps growing and there a lot of these messages in the logfile:

"zabbixserver Zabbix server[16994]: Zabbix agent item [system.cpu.util[,iowait]] on host [anotherserver] failed: first network error, wait for 15 seconds"

After I restarted the zabbix-server service the amount of items in the queue decreased quickly and it is collecting data again. I couldn't find the problem, yet. Is it a bug?