I used to have category pages like this
Then i installed the seo plugin and activated it to remove the /category/ in the url from category pages.
i changed WP-permalinks back to default and resaved them but the plugin is adding the wrong canonical tag for category pages now.
check the source on this page (row 17).
As you can see the canonical link is pointing to the old url with the /k/ in the middle and this is really bad. It seems to be saved somewhere and i cannot find where.
i have tried reactivating the plugin, resaving every options page, resaving permalinkpages but the canonical wont change.