Fix XML sitemaps to work by forcing .htaccess to pick them up. Add this to the .htaccess, above the WordPress rewrites. This version assumes you're using a subfolder install with WordPress installed in /wordpress/, adapt if needed.
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|# WordPress SEO - XML Sitemap Rewrite Fix|
|RewriteRule ^sitemap_index.xml$ /wordpress/index.php?sitemap=1 [L]|
|RewriteRule ^([^/]+?)-sitemap([0-9]+)?.xml$ /wordpress/index.php?sitemap=$1&sitemap_n=$2 [L]|
|# END WordPress SEO - XML Sitemap Rewrite Fix|