Sitemap error in search console

[Nuts]

I keep seeing this error in google search console: Your Sitemap or Sitemap index file doesn't properly declare the namespace. Expected: http://www.w3.org/1999/xhtml Found: http://www.w3.org/TR/xhtml11/xhtml11_schema.html,  Parent tag: url, Tag: link...

How do I fix this error ? Thanking in advance for any advice.

[datahell]

Do the following and tell me if the problem is solved:

open this file:
components/com_sitemap/controllers/base.php
go to line 45:
$this->alternate = true;
and comment it:
//$this->alternate = true;

Delete any cached sitemaps (repositoy/cache/) and re-check it.