500 error when login

This is only happening with the Joomla login correct? Is Facebook login working correct? I've never seen that issue before and don't think it's related to JFBConnect at all. I'll provide what help I can to help you debug it, but don't have any real solution.

Please do each of the following and test the login after each one to see if any helps the problem:
* Disable your SEF URLs. It looks like you're just using Joomla's SEF feature, but if you're using a 3rd party extension, disable that as well.
* Try a different template. Your current template has a custom 500 error page which is hiding what the error is. Seeing the real error will help understand the problem.
* Disable any caching you may have enabled.
* If you're not using the Joomla Login module, try enabling that module and use it. It should be the same as our SCLogin module (if you're using that, I can't tell), but the Joomla one should definitely work.

Let us know how that goes, and good luck!
Alex