Whenever a Joomla article or category is created, the system records the account that created it. This is useful for content items where we also want to display a link to the user who wrote the article, or for displaying content authored by a specific Joomla user. 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.
Note that this message is a warning rather than an error, so while it can be annoying and may show notices or warnings it won't break your site. In the future, when users leave your organisation it is better practice to disable their Joomla account rather than to delete it. That prevents this issue from arising again.