Core output, SEO plugin and theme head
Open the Canonical target and compare its current permalink with a saved SEO override. A renamed contribution can leave an old fixed value.
On a single content page, check if WordPress Core, SEO plugin, and theme each output canonicals. For archives, a different logic may apply than for contributions. Do not blindly remove the core function, but first determine the leading source.
Where to check the setting in WordPress
Open the head of a published single view and search for rel=canonical. Assign the output to the core call rel_canonical, the active SEO plugin or your own theme snippet. A second manual output in the header does not fix an incorrect plugin value.
Example of this specific finding
Illustrative control case: /article/? ref=mail refers via canonical to /alter-article/, the target answers with 404. The preferred address no longer contains the intended article.
follow-up audit in WordPress
Compare a post, a static page and an archive. The additional tag comes from a theme snippet, the change of which is first checked in a test environment.
Determine the appropriate achievable goal and correct the canonical source. Then retrieve the source page and destination separately.
Derogations and scope
Any achievable goal is not a substitute. If the article no longer exists, first clarify the publishing state instead of just the canonical field.
Related: WordPress with classic or block theme. SEO plugins and page builders are different; there is no universal menu path for all installations.
Classify the general error
Definition, system-independent causes and further correction examples: The preferred target must have the intended content