It is not preventing the sitemap from working. The sitemap.xml is loaded just fine. It's not even a real file, it's a route in WordPress which always works. What's blocked is the XSL file which is used when accessing the sitemap.xml over a browser to convert it to human-readable HTML.
Go to Admin Tools, .htaccess Maker. In the “Files which will always be made accessible” area add
wp-content/plugins/wordpress-seo/css/main-sitemap.xsl
and click on the Save and Create .htaccess button at the bottom.
Nicholas K. Dionysopoulos
Lead Developer and Director
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