if you were coming from FormBase.aspx to login.aspx, how could you get to default.aspx? how did you do your redirection after authentication?if you want to go to FormBase.aspx specifically in login.aspx, use
Response.Redirect("FormBase.aspx");
Response.Redirect("FormBase.aspx");
<authentication mode="Forms">
<forms name=".theDefault"
loginUrl="~/login.aspx"
path="/"
protection="All"
timeout="15" >
</forms>
</authentication>
<authorization>
<deny users="?" />
</authorization>
</system.web><location path="FormBase.aspx">
<system.web>
<authorization>
<allow users="*" />
</authorization>
</system.web>
</location></configuration>