If you see the error "JUser: :_load: Unable to load user with ID:" while revising an article, this means the article has been inadvertently assigned to an unknown or deleted user. This commonly happens from upgrading Joomla from a very old version to a newer version.
1.) Select the "Publishing" tab
2.) Click the Person Icon to select a user.
3.) A pop-up will appear. Choose yourself or another admin who is listed.
4.) Save the page.