skittery on "[Plugin: Yoast SEO] Can't generate xml sitemap"

I can't seem to create an XML sitemap.
I know it's something on the site and not a bug in the plugin.
When pressing generate i get to a 404 page.

Could you possibly tell what the problem could be?
Maybe an htaccess issue?

Thanks!

https://wordpress.org/plugins/wordpress-seo/