There's a couple of likely issues:
* First, I'd recommend upgrading to JFBConnect v6.1, as there are multiple improvements.. however, the older release isn't likely the cause.
* Next, check if you have the JLinked System, User and Authentication plugins still enabled. If so, disable them. They are no longer necessary since you're using JFBConnect.
I hope that helps, but if not, as mentioned in the other post, please PM us details and we'll investigate further.
Thanks,
Alex