iTunes rejected my podcast feed URL

Ah I see. That is actually a bug in SermonSpeaker which has been fixed long ago.
Are you using the latest version available? Because at least in SermonSpeaker 4.5.8 that should already be fixed.

Yes, I am using 4.5.8.
The sermoncast module is 4.5.1, however, because I believe that is the latetst compatible version with 4.5.8?

The SermonCast module only provides a link. It doesn't do anything with the output itself. That's coming all from the SermonSpeaker component.

I don't know why it doesn't work for you then. I just tried it myself with a test installation and it doesn't show me that warning when passing through the validator.
Maybe a plugin does set wrong document headers then in your site?

Okay, thanks. So is it a case of identifying the plugin that is causing the problem? If so, do I have to just disable them one at a time and then run the validator each time?

That's what I would start with, yes.
Disabling all non-core plugins and see if one of them is the cause by enabling them one by one again.
There is a little trick in how to know which plugin is core or not. Just order them by "ID", the non-core plugins have an ID 10'000 and higher.

Ah, great tip! Thanks

Unfortunately, I disabled all the non-core plugins (which were just Acy Mailing related), but that didn't make any difference with the validator