Linux log file monitoring by zabbix 2.2.1

It might be an access issue - Zabbix typically runs as an unprivileged user and if the log file in question is not readable by it, then you will get that error.

If that isn't the case, I suggest you enable loggin on the agent and examine the log - this usually provides useful information.

The "log" agent item checks the whole log by default, you must tell it to use the "skip" mode to avoid processing of older data. Simply add a ",skip" to the end of your parameters to the log item. Check carefully this page in the documententation.