If this happens for users that registered using Joomla, and not a social network, that is an indicator this is not related to JFBConnect and more of an underlying Joomla issue. JFBConnect does not attempt to access that table at all.
Our best guess is to look at any other user management or profile extensions you have on your site, and systematically test disabling their user, authentication and system plugins to see if you can identify which is causing the problem. Likewise, a quick way to guarantee that it's not JFBConnect is to disable the 'Authentication - JFBConnect Social Integration', 'System - JFBCSystem', and 'User - JFBConnect Social Integration' and then try to log in via a Joomla account.
-Melissa
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