Getting "Page Not Found" from content type after upgrading to 8.9

Hi,

I upgraded my site from 7.11 to 8.9. Everything seems fine except for a custom type called Article. When one clicks this content, it returns "Page Not Found". It only seems to affect this content type. When I find the articles in the admin area, I can see the content and setting, but it will fail in the preview and when I navigate to the page.

Any ideas on how to troubleshoot this?