[Plugin: SEO by Yoast] ERROR: sitemap_index is not a valid feed template.

ERROR: sitemap_index is not a valid feed template.

An old problem, and I’ve read past posts. It’s a very simple web site using the Twenty Eleven theme v 1.5.

I’ve hunted down the only place where the following appears:
'(.+).xml' => 'index.php?feed='. $wp_rewrite->preg_index(1)

In the file mf_gig_calendar_feed.php, part of the very nice MF Gig Calendar plug in.

I commented it out, then deactivated the MFGig Calendar plugin, but am still getting the same error.

The site is at https://www.susanevanspiano.com
The site map should generate at: https://www.susanevanspiano.com/sitemap_index.xml/