It seems like the Modal Javascript framework, that's a part of Joomla, isn't able to be initialized properly. We use that to display the "You are now logging in" popup message. Can you disable that feature in the Configuration area of JFBConnect and try again?
I'm not seeing anything about an xd_arbiter.php error. The issue above that I describe seems like a simple Javascript error that's preventing further execution.
If that still doesn't work, please let us know what browser and OS you are using.
Thanks,
Alex