IAuthenticationService.Login continue to show "The login credentials supplied are invalid"

   Hello,
I need to add to my wpf application a login function... I've generated on a test DB the aspnetdb using aspnet_regsql.exe, I've added in my Caliburn micro bootstrapper those lines (in the Configure method)

var securityRepository = new SecurityEntities();

var admin = new AuthenticationService<SecurityEntities>(securityRepository);

and in my loginviewmodel I've the Login method

public void Login()
{
    var loginCredential = new LoginCredential(_userName, _password, null);
    bool isLogged = _authenticationService.Login(loginCredential);
}

I've created as imple user in the aspnetdb using the stored procedure that's Paolo/paolo ... but I continue to receive "The login credentials supplied are invalid."

My web.config (for the server part is)

<ideablade.configuration version="6.00" xmlns="http://schemas.ideablade.com/2010/IdeaBladeConfig">
    <logging logFile="log\DebugLog.xml" />
    <objectServer>
      <serverSettings loginManagerRequired="false" useAspNetSecurityServices="true"/>
    </objectServer>
    <!-- Additional configuration can be added to override defaults. 
         See the sample config files in the Learning Resources for more information.
    -->
  </ideablade.configuration>
...
  <system.web>
    <compilation debug="true" targetFramework="4.0">
      <assemblies>
        <add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
      </assemblies>
    </compilation>
    <authentication mode="Forms"></authentication>
  </system.web>

What am I doing wrong?
Thanks
Paolo