I've submited this as bug 20967.

The spec for RSS 2.0 says that for each item you need at least one of title or description tags. ZCS RSS reader requires the description tag. To test you use the included rss. Removing the title tag for the item, the feed works. Keeping the title but nuking the description tag makes the item not visible.

The spec can be found at RSS 2.0 specification and the
relevant part is "All elements of an item are optional, however at least one of
title or description must be present."

<rss version="2.0" >
<channel >
<language >en-US</language>
<copyright >foo</copyright>
<description >Test</description>
<title >A Test RSS</title>
<link >http://zimbra.com</link>
<item >
<guid isPermaLink="false" >some-id</guid>
<title >Test Title</title>
<link >http://zimbra.com</link>
<description >Test Description</description>
</item>
</channel>
</rss>