Hi,
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
#__users table and search for the ID of the user you just created
- Change the
ID for this user to the one you see in the Warning message. ID: 871
And save
It will no longer give a user id error.