SeoPress has updated the sitemap configuration to make it better in the eyes of Search Engines. But you need to add rewrite rules on servers that use Nginx.
You can copy the following code into your Nginx Server.
location ~ ([^/]*)sitemap(.*)\.x(m|s)l$ {
## SEOPress
rewrite ^.*/sitemaps\.xml$ /index.php?seopress_sitemap=1 last;
rewrite ^.*/news.xml$ /index.php?seopress_news=$1 last;
rewrite ^.*/video.xml$ /index.php?seopress_video=$1 last;
rewrite ^.*/author.xml$ /index.php?seopress_author=$1 last;
rewrite ^.*/sitemaps_xsl\.xsl$ /index.php?seopress_sitemap_xsl=1 last;
rewrite ^.*/([^/]+?)-sitemap([0-9]+)?.xml$ /index.php?seopress_cpt=$1&seopress_paged=$2 last;
}