Twenty Eleven 3.1 and Yoast 9.5 sitemap issue

Sitemap generated by yoast (https://domain.com/sitemap_index.xml) gives an HTTP ERROR 500
It happens only with Twenty Eleven theme. Switching to another theme solves the problem.

Wordpress 5.0.3
PHP 7.0
All plugins and themes updated
All other plugins except Yoast were disabled in order to check conflicts

Adding this code didn’t help:
/* Limit the number of sitemap entries for Yoast SEO */
function max_entries_per_sitemap() {
return 100;
}
add_filter( ‘wpseo_sitemap_entries_per_page’, ‘max_entries_per_sitemap’ );

Saving permalinks page without changing any of the settings also doesn’t help