Podcast download button is downloading 0kb file

The Podcast download button on my website is downloading a 0kb file with the correct filename on my site.

A link to the latest podcast on my website is:

https://www.floorislava.com.au/podcast/ep-12-the-giant-black-curtain-of-doom/

I’ve hidden the download link so that my viewers don’t try to click on it. The code is still there if you use an inspector – it is directly below the media player. The download link for that podcast is:

[audio src="https://www.floorislava.com.au/?podcast_episode=https://www.floorislava.com.au/podcasts/12-minimum-requirements_episode-12.mp3" /]

But if you go to that link, you only get a 0kb file. You can play the podcast via the media player which verifies that there is nothing wrong with the file.

I am running some plugins, notably Redirection and W3 Total Cache but the problem was still occuring when I disabled them so they are not causing the issue.

https://www.ads-software.com/plugins/seriously-simple-podcasting/