Can you check the CB configuration for user fields and see if any fields are marked as "Required" that aren't being filled out during the registration process using JFBConnect? if there are *any* fields in CB marked as 'required', they must have a mapping configured in the Profiles area of JFBConnect or be editable by the user on registration. Otherwise, CB will not allow us to save the user.
Check every field in CB for the required setting and test disabling them all to see if that fixes the problem. If so, hopefully that gets you started, but if not, we'll help how we can.
Thanks,
Alex