Warning JUser: :_load: Unable to load user with ID

If after using Joomla QuickStart package you’ve deleted all the “demo” users – the Joomla! will have a problem to associate previously-created content or added items to the user, which no longer exists. The warning message “JUser: :_load: Unable to load user with ID: XX” is telling us that the user with the XX id number could not be loaded, typically because that user has been deleted.