Hello,
in _blog_main.inc.php file, on line 213, there is a comment saying :
"// Also use the prefered posts per page for archives (may be NULL, in which case the blog default will be used later on)"
speaking of posts per page on the archive page.
This does not work. When using NULL, archive default (100 in my case) is used,
whereas when using 0, blog default is used.