If there have been any lockouts the log is show on the bottom of the plugin settings page.
I assume you get that error with version 1.4.1 of the plugin (as that is the only way the line number makes sense).
Do you use PHP 5?
That code is about adding an IP to the lockout log.
Do you get the error reliably? If so it should happen any time a new IP/attempted user combo is to be logged.
Does the log work for you? I assume from your first question that it might not.
I think I know what might be the problem. Would you be able to check a modified version of the plugin? If so, please mail me at <email@example.com>.