This has been known to happen if you modify the forum hierarchy, like move a wiki forum to another parent forum, if you change the name of a namespace, and other sometimes random causes. Actually what happens is that the articles are no longer in sync with the forum system.
You can resync your articles by going to AdminCP > VaultWiki > Namespace Manager. In the row for your main namespace, click "Rebuild". If you notice this happening in other wiki forums like Templates, or Images, you may want to just re-sync all pages:
AdminCP > VaultWiki > Wiki URL Manager > Rebuild Page URLs.