Anonymous Authentication is turned on by default when using Forms based
Authentication. Make sure you have an Authorization element in the
web.config where the asp.net form  is that looks like the following:



<authorization>

<deny users="?"/>

<allow users="*"/>

</authorization>



With Anonymous Access turned on the request will reach the site, the deny
entry above will deny Anonymous Users access, the Allow above will allow
Authenticated users access. Since Anonymous Access users are denied, the
user will be redirected to the logon page. The order of the entries is
critical. If this Authorization element isn't in the web.config FBA won't
work. You should also look for a <Location> element that points to the
specific virtual  directory where the file is. If that is present and has an
Authoriztion element it will take precedence.