I was currently using All-in-one-SEO in my blog, but looking for a solution to send also sitemap to Google News I cam across with Yoast SEO plugin, which is great, but I found this bug when trying to configure
The problem was:
Notice: Undefined index: wpseo_sitemaps
It breaks your blog installation
This is caused when you have unticked the option to generate the XML sitemap. I had to uninstall the files, so the plugins were no longer recognized and had to reconfigure it.