I've also noted some irregularities. To verify a site, Google will ask for a unique filename, such as GOOGLE11e5844324b7354e.html. I have noted, however, that Google bot asks for google11e5844324b7354e.html and naturally gets 404 error because that filename is case-sensitive. What's wrong with sitemap recently? Why Google sitemap bot would ask for the lower case version of this file?
joker197cinque beat me to it. Please ignore the following:
That is probably because you have a custom 404 page in .htaccess. Try temporarily renaming the .htaccess to something else, click on sitemap verify, wait till verified, rename the .htaccess back to itself. During the renaming your site might be unavailable.