sue_ on "Archived posts showing as 404 error"

I have searched the forum and cannot find what I need to cure a problem.

It seems I am getting 404 errors recently on archived posts that actually exist.

Here is the post normally ---> link

And here is the post with the 2013 archived bit showing 404 ---> link

This is something new happening. Could someone please tell me how to fix this.

Many thanks