"first network error"

Currently I'm getting the following message for a *nix host:

21081:20111013:102745.243 Zabbix Host [xxx]: first network error, wait for 15 seconds

Probably not a big issue, just some item that can't be retreived. However, in the interface everything looks ok and no unsupported items. I'd like to resolve this, but I don't know which item is causing this issue. Please include an item name in log message.