I just tried to register on your site and got the error you mentioned. I've never seen that before, and there aren't any known issues that I can find with Joomla regarding that. It's not something from JFBConnect, but we'll try to help how we can.
Looking at that line of code, it seems to be a problem loading the component parameters for the User component of Joomla, specifically, the user's language. Can you go into the User Manager, hit "Options" and simply hit "Save". Then, test again. If that doesn't work, please go into the Language Manager, select a language and hit the "Default" button. Then, again, test.
Hopefully, one of those help get you going. There's no reason that changing the Error Reporting to "Simple" should 'fix' the issue, it may just prevent the pretty error from displaying. I would still expect a blank white page to appear.
Let us know how that goes. Until we can get in, it's difficult to diagnose the other problem, but this issue seems more important anyways.
Thanks,
Alex