Do not disable "User - Joomla!" user plugin as that's for Joomlas core user and is necessary. If you don't have a 3rd party user plugin enabled then it's likely a 3rd party system plugin since they can act on any Joomla event. Just doing a quick Google search leads to similar issue below.
www.easy-profile.com/support/duplicate-entry-principals-peter-chielens-for-key-uri-message.html
Which points to "System - DPCalendar". Do you have such a plugin present in Extensions > Plugins? If so disable it and retry. Sorry, I can not help you any further as your issue has nothing to do with CB.