Visit your Settings, Permalinks in the WordPress Admin. You don’t have to make any changes, just visit the Settings page and hit Save. This should correct the issue.
If it keeps cropping up, that means you’ve got something else in your theme or in other plugins that is messing with rewrites. You can look through your site to see if your theme or any other plugins are calling `flush rewrite rules` which is the usual thing if it’s called improperly that will give you 404’s.
To look for plugin conflicts, use our Theme & Plugin Conflict FAQ.