Welcome the Technical Support section. Help us in assisting you by providing us with a concise and descriptive elaboration of your issues. Be specific and if possible, provide us with a step-by-step instruction in replicating your problem.
I just tried to register and then login using FBC on this site, and everything seems to work just fine. So the problem is also occuring randonly this site?
I'm not able to replicate this, so could you maybe inspect your site with firebug and check if there's any javascript error while trying to login via FBC ?
There's not much I can do to debug this problem since it's not occuring for me, so I really need a screenshot of firebug console with javascript error. This should be very helpful for finding a cause of this issue.