The following xml contains a unescaped "&", which is not valid:
<author>
<name>Bulma: Free, open source, & modern CSS framework based on Flexbox</name>
<email>bbxdesign@gmail.com</email>
</author>
This change should fix it.
<id>{{ site.url }}</id>
<updated>{{ site.time | date_to_xmlschema }}</updated>
<author>
- <name>{{ site.data.meta.title }}</name>
+ <name>{{ site.data.meta.title | xml_escape }}</name>
<email>bbxdesign@gmail.com</email>
</author>
<atom:link href="{{ site.url }}/atom.xml" rel="self" type="application/rss+xml" />