Still investigating the exact cause, but the problem appears to be in Facebook's Javascript button. In the SCLogin module, if you switch the "Login Button Type" to "Image" instead of "Javascript" and then use the Image field to set your own custom image, that works. Using an image uses our own Javascript and bypasses alot of Facebook's code and, again, seems to work.
You will need your own custom image. Feel free to copy ours *to your server* and use that. Please don't link to it directly:
www.sourcecoast.com/sclogin/features/
Hope that helps get you around the issue for now, and rest assured, we'll be looking into the underlying cause more and reporting it to Facebook if it's indeed a bug on their end.
Thanks,
Alex