This sometimes happens when a specific user has been deleted that was the author of an article. When trying to load that content, it also tries to get the username based on the ID, which cannot be found in the #__users table.
- Create a new user via the Joomla User Manager (admin backend).
- Open your database manager using PhpMyAdmin
- Go to the
#__userstable and search for the ID of the user you just created - Change the
IDfor this user to the one you see in the Warning message. e.g. ID: 42 - Go to the
#__user_usergroup_maptable - Search for the
user_idthat matches the ID of the user you created before - Change this also to the one you see in the Warning message. e.g. ID: 42
You can also run the below query via PhyMyAdmin and it should do the job for you. Note you must run the query after you are done creating the new user.
UPDATE `#__users` SET `id`= ‘NEWID’ WHERE `#__users`.`id`= ‘OLDID’;
UPDATE `#__user_usergroup_map` SET `user_id`= ‘NEWID’ WHERE `#__user_usergroup_map`.`user_