Error in podcast feed

The odd thing is that the problem seems to be caused by the "Facebook Like and Share" Content Plugin which comes with Joomla!

It doesn't seem likely that a plugin shipped with Joomla! doesn't use the Joomla functions, does it?

I posted something to the Joomla support forums, but no response so far:
forum.joomla.org/viewtopic.php?f=628&t=688052

what you could do is add the following line in code for that facebook plugin. right after the line

Bob wrote: The odd thing is that the problem seems to be caused by the "Facebook Like and Share" Content Plugin which comes with Joomla!

It doesn't seem likely that a plugin shipped with Joomla! doesn't use the Joomla functions, does it?

I posted something to the Joomla support forums, but no response so far:
forum.joomla.org/viewtopic.php?f=628&t=688052

You're right in that it wouldn't be likely. But to my knowledge there is no such plugin shipped with Joomla!
Maybe you got a special Joomla! package with added extensions from another site, but those are not part of the core Joomla found on Joomla.org.

Wow - thanks Andrei!

Although I'm a developer in my other life, I'm not a Joomla developer. In fact I'm a Joomla newbie/ignoramus. Is it possible to make this change directly to a Joomla installation? Can you give me a more specific pointer to the file I'd need to modify?

Thomas - thanks for correcting me. I incorrectly thought that anything found in the Joomla! extensions directory (e.g. extensions.joomla.org/extensions/social-...facebook-share/16602 ) was part of Joomla, but it appears that's just a broker for third-party content. My partner must have found and installed this extension.

I'll share Andrei's updates with the proper folks.

You guys rock! Thanks so much.

The JED is the directory where anyone can register their extension. In fact SermonSpeaker is also listed there. It's quite nice

Also to answer your question on the Joomla forums how an error in a plugin can break our feed: Joomla runs the system plugins always, on any page generated. The content plugins are called by SermonSpeaker for some fields (like scripture and comment field). What those plugins do when they are called is up to them. Usually they search for special tags and replace them with their own HTML or Javascript code. Now this works all fine if the document is a HTML document, which it is probably 99,99% of the time. But since we try to deliver an XML file, any HTML or Javascript code in it may break it.
I can work around it by removing all HTML tags, that is no problem and is what SermonSpeaker already does. Also Joomla offers a special command to include Javascript into the HTML header, which then isn't rendered in our XML case.
Now if the plugin doesn't use this functions or doesn't check for the document type, it may break our XML.
Also if a plugin file contains some UTF-8 BOM, it may also break the output because that BOM is found in the generated code (usually a "space" at the start).

In case of a Facebook plugin, the change Andrej wrote should be the correct way to do it. We need a Facebook like button on HTML documents only anyway
If the plugin follows the Joomla rules, the file should be in /plugins/content/nameofplugin/nameofplugin.php.

I applied Andrei's fix, and now the feed validates. Thanks amigos!