Just try to update permalinks (i.e. Trigger a flush of the rewrite rules). More Solution Advice on setting up a site with front end registration in WordPress query posts - Trouble with get_next_post() get_previous_post() returning null in WordPress admin - Getting Rid of the WordPress.com Toolbar on Other Sites? in WordPress plugins - Qtranslate displays empty categories with get_categories() in WordPress menus - Placing Admin Post Metaboxes in Tabs in WordPress Reffered: https://wordpress.org/
