FormsAuthentication.Authenticate() method is not working

Hi,

I have a login page (Logon.aspx)and a default page(Default.aspx).

My Web.config files authetication and authorization section looks like:

<system.web>
  <authentication mode="Forms">
   <forms loginUrl="Login.aspx" protection="All" timeout="30" name=".ASPXAUTH" path="/" requireSSL="false" slidingExpiration="true" defaultUrl="default.aspx" cookieless="UseDeviceProfile" enableCrossAppRedirects="false">

    <credentials >

     <user name='a' password='b'/>
    </credentials>
    </forms>
  </authentication>
  <authorization>
   <deny users="?" />
   
&nb ...