WordPress creates an array of rewriting rules which is responsible for administering the permalinks (or friendly URLs).
Plugins poorly programmed types of custom publications and changes in the taxonomy can spoil the array containing the permalinks, causing 404 errors.
Quick fix: regenerate permalinks
Permalinks arrangement can be regenerated by going to the “Permalinks” tab in the WordPress admin panel, and simply restore the structure of permalinks.
WordPress Dashboard -> Settings -> Permalinks: Click “Save Changes”.
Now test your URLs. If a 404 error is no longer displayed, it means that something caused a problem with the structure of permalinks, but since it was repaired.