Troubleshoot IIS authentication

Is there a reason why Forms authentication works fine using project debuging but not with IIS 7.5 ? In my IIS auhtentication I only got Anonymous and Forms enabled. check my authentication part of my web.config file, the logon page should redirect to FramePage.svc after validation. FramePage.svc permision is set to everyone. it seems like its not happening, any reason for this ? I'm getting error "Unknown exception occurs while logon to system, check with your admin. And this is if enter the right credentials. If enter wrong username, I will get invalid credential which is normal. Appreciated.

<authentication mode="Forms">
        <forms name=".Global" defaultUrl="FramePage.svc" loginUrl="LogOn.aspx" slidingExpiration="true" timeout="60" />
    </authentication>