I get this error in the Google search console / webmaster tools about a serendipity sitemap:
Sitemap can be read, but has errors
Unbound XML namespace prefix.
The XML Sitemap cannot be parsed because it contains one or more unbound namespace prefixes. For example, this error is generated when <xhtml:link> is found in a Sitemap without prior xmlns:xhtml="http://www.w3.org/1999/xhtml".
I can reproduce this on two different s9y installations.