WPMU DEV SEO - Sitemap problem

Hi, i'm running 2 multisites with wpmu dev seo plugin, on one of them i have an error message in the header, about sitemap.
Warning: file_put_contents(/homez.386/site/www/wp-content/uploads/sitemap.xml) [function.file-put-contents]: failed to open stream: No such file or directory in /home/site/public_html/wp-content/plugins/wpmu-dev-seo/wds-files/wds-sitemaps/wds-sitemaps.php on line 131

What's the problem?