1/ The page-sitemap generated is empty... And the plugin redirect to a 404 not found.
I think one of these may be better when it's empty :
Not having page-sitemap in sitemap_index (keeps Not found error)
Having an empty page-sitemap (doesn't seem the best idea ^^')
2/The page-sitemap generated empty shouldn't be empty.
For Post and Page : The error occurs when the option "Include in Sitemap" is to "Auto detect", instead "Always include" should work.
"Auto detect" works if in the advanced settings you have "Meta Robots Index" to "Default for Post type, currently : Index"
If you have "Index" it doesn't work.
Seems like it's in the file inc/class-sitemaps.php : what seems to work is changing the line 326
if ( wpseo_get_value( 'meta-robots-noindex', $p->ID ) && wpseo_get_value( 'sitemap-include', $p->ID ) != 'always' )
if ( wpseo_get_value( 'meta-robots-noindex', $p->ID ) == 1 && wpseo_get_value( 'sitemap-include', $p->ID ) != 'always' )
Seems like 1 is NoIndex, 2 is Index and empty is default.