A host was rebuilt and due to the period of time before the agent was reinstalled, the server marked the host with a connection error of Interrupted system call. This error condition is impossible to clear, there is no way to force the server to try to talk to the agent again. The only way to get this working was to delete the host record entirely, lose all historical data, and rebuild the host entry. This is not desirable. There should be a way to tell the server to try again. This sort of behavior has been observed in several different cases, and in every case if there was some way to tell zabbix to clear the error condition and try again we'd be back in business rather than rebuilding from scratch. This has been a stumbling block in every version of zabbix from at least 1.8. I call it a bug because of the impact and failure condition.