The problem looks like you have Admin Tools plugin installed with the option to force the website running on https, so it replaces every links encountered in the Joomla! output with the https protocol version.
Indeed the correct link for the namespace of an XML sitemap document is:
and if it's replaced with the https version it won't be valid:
The problem derives from the setting named 'Convert all links to HTTPS when site is accessed over SSL' of Admin Tools. Ensure that it's set to 'No' to fix the problem.