This is often caused by an issue with permalinks.
Go to Settings > Permalinks and re-save the Permalinks page. (Do not make any changes to the settings. Just click the blue button to re-save the current settings.)
Sometimes when new custom post types, such as AdSanity's ads, are activated on the site, the permalinks do not reset to include the new type. This solution should reset the permalink structure and include AdSanity in the process. This will not affect anything on your site (unless you make changes to you permalink settings, in which that will have obvious implications).