Here's the details
Permalinks set up as category/postname
Only happens in one category
Only happens from "outside" admin
When I click on or type in the full URL in a browser window I get a 404 error. When I'm in Admin edit mode and click on view, the post is resolved fine. And, it only happens for one category (the default I renamed from "uncategorized"). All other categories I added are fine.
I've tried deactivating all plugins except those that are absolutely required. Nothing seems to change. Yesterday I upgraded to 2.9.2 hoping for a resolution, but it's still broke.
How can I possibly debug this?