No, I'm not using Application.cfc and I doubt its timing out so soon.

Login.cfm code:

<!--- Checking the login --->
<cfif isdefined ("form.username")>
<cfset good=1>        
    <cfquery name="checklogin" datasource="mydata">
        select username,pw
        from login
        where username='#form.username#' and pw='#form.pw#'
    </cfquery>
<!--- Set a session and updating visitors count --->
<cfif checklogin.recordcount is 1 and Compare(form.pw,checklogin.pw) Is 0>
<cfset session.username='#checklogin.username#'>
<cfquery name="updatelogin" datasource="mydata">
    update login
    set lastvisit=#now()#, visits = visits + 1
    where username='#session.username#'
</cfquery>
<cfelse>
    Incorrect login.
</cfif>
</cfif>

Logout.cfm code:

        <cfset tmp=StructDelete(session, "username")>
        <cflocation url="/login.cfm?lo" addtoken="no">

Error Message:

Variable SESSION is undefined.