setAttribute: Non-serializable attribute error

I have an application that was developed using netbeans 5 IDE and was working up until today. Now it generates the following error:

SEVERE: java.lang.IllegalArgumentException: setAttribute: Non-serializable attribute

javax.faces.el.EvaluationException: java.lang.IllegalArgumentException: setAttribute: Non-serializable attribute

The JSP code which causes the error is:

<h:outputText value="#{msgs.NotLoggedIn}"

rendered="#{! Login.loggedIn}"/>

The Java code being called is:

private boolean loggedIn = false;

public boolean isLoggedIn( ) {

System.err.println( "JsfLogin.getLoggedIn called" );

return loggedIn;

}

The application still works when built under netbeans 4.1.

What does this error mean and what could be causing it?

[818 byte] By [reichewa] at [2007-10-3 8:47:28]