Sitemap.xml cannot change name - Path “/ sitemap_es.xml” is not available and can not be used

I also ran into that, and there’s a simple reason, and a simple and clean solution, which doesn’t tap any code.
They simply limit the file naming to this pattern: “/*/sitemap.xml”. So you can choose any directory, but you’ve to name the file “sitemap.xml”.
That implies, that you create an own directory for every sitemap you generate!
But regardless how simple the solution might be, the question arrises, WHY it is like that at all?! I can’t think of any reason, and it’s not even mentioned anywhere in the admin interface, so of course, you’re confused and start checking path permissions, etc.,
This is even the case with the latest 1.6!
Sourcinggate’s modification is not a very good idea, as it makes a maybe also otherwise used verification function to always return true, so you might mess up paths somewhere else, and Magento can’t tell you anymore.
So, here’s the fix:
In

/app/code/core/Mage/Adminhtml/etc/config.xml

Replace (at around line 190)

<any_path>/*/sitemap.xml</any_path>

with

<!-- zLabs | Black | BEGIN -->
<!-- No wonder, that we couldn't save any other filename than sitemap.xml ;)
        Is this a bug, or a "protection" feature? It doesn't tell this anywhere
        in the admin interface ... -->
<!-- <any_path>/*/sitemap.xml</any_path> -->
<!-- zLabs | Black | END -->