Client Application Services Membership.ValidateUser Always returns false

Question

I have a WPF application using Client application services to validate a user against a web site Default SQL Membership provider.

No matter what I do, the Membership.ValidateUser(username, password) on the client returns false.

I even changed the ServiceUri to an invalid URL and it stills returns false rather than throwing an exception.

This is the Client application config

 <system.web>
    <membership defaultProvider="ClientAuthenticationMembershipProvider">
      <providers>
        <clear />
        <add name="ClientAuthenticationMembershipProvider" type="System.Web.ClientServices.Providers.ClientFormsAuthenticationMembershipProvider, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" serviceUri="https://My-PC/Server/Authentication_JSON_AppService.axd" savePasswordHashLocally="False" credentialsProvider="" />
      </providers>
    </membership>
  </system.web>

And this is the Server's web.config

  <system.web.extensions>
    <scripting>
      <webServices>
        <authenticationService enabled="true" requireSSL="true" />
        <roleService enabled="true" />
      </webServices>
    </scripting>
  </system.web.extensions>

Note: the web site is using SSL (dont know if this is relevant or not).

What can i do to check what is the error? or how to get this to work?