Recent Posts widget doesn't work.

It’s not the widget. The permalink for this post doesnt work: https://davidecrowley.com/the-rotten-apple-tree/

try

– deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s).

Did you edit the files of the default theme Twenty Twelve?

I deactivated all plugins without resolving the problem. I haven’t edited any default theme files.

Here’s how the permalink looks from the editing page for that post:
Permalink: https://davidecrowley.com/the-rotten-apple-tree/ ?

Thanks for helping

Do you have a page or post with the same title (slug) in the trash. If so delete it permanently from the trash.

There was a page of the same name in the trash which I deleted permanently. Problem fixed!
Many thanks.