Hi,
Problem is not sitemap, i think problem is in .htaccess file
It may be a rewrite rule that is rewriting the URLs for 404 pages and solution is to move (rename) your .htaccess file for a second and try to verify your site with the file method again. Then return (change) .htaccess back to its normal state.
It may be a custom 404 page which using the apache software error pages is actually displayed with 200 OK status and solution is to remove the “404 custom page” and try to verify your site using the file method again.