Hi Ann,
Your Custom 404 extension is still out of date, you can check the latest supported versions here
You can deactivate and delete your current version (1.1.3), then navigate to X > Validation > Extensions and reinstall the Custom 404, navigate back to plugins panel and activate the Custom 404. Because all of the plugin settings are stored in the database, and because version 2.x will migrate settings from older versions, its safe to use this method of deleting the previous version. But you might still need to adjust a few options and re-save the custom 404 settings. Also, re-save your Settings > Permalinks
If you have any caching plugin/features on your site, please clear it.
Let us know how it goes,
Cheers!