If you're the administrator of the Facebook page, log into Facebook (in another tab) with that user. When you click the Login With Facebook button, it should give you a more descript error message that's only shown to administrators.
Most of the time, when this occurs, it's because your application isn't setup correctly though. In the app settings, if your website is
www.site.com, make sure the app is configured as follows:
Site URL:
www.site.com/
Domain (or Base) URL: site.com
Hope that helps get you fixed up, but If that doesn't help, please post your URL, and we can take a look.
Thanks,
Alex
Facebook Connect stopped working
