[Plugin: WP Download Codes] Broken Permalink?

I’m trying to use a download code in a post but it seems to have done something weird to the permalink.

It’s the first post here:

https://cmx.org.uk/

The permalink is supposed to be this:

https://cmx.org.uk/5920/

But this returns a page not found error. I had the post as private for a while yesterday while I was setting it up but it’s now public (don’t know if that makes any difference?)

Any clues?

https://www.ads-software.com/extend/plugins/wp-download-codes/