Wordpress SEO by Yoast vs Simple Sitemaps for Multisite

hi there,
i had the simple sitemap for multisite network activated and i'm also using xml sitemaps functionality which comes with the Wordpress SEO by Yoast (it's allowed per blog). unfortunately i've recently started getting 'ERROR: sitemap_index is not a valid feed template.' message. i've been trying to disable all other plugins (including 'simple sitemaps for multisite' but it didn't fix the issue.
i'm actually wondering whether i need that sitemap functionality by yoast at all while i'm using your plugin network activated anyway.
any ideas?
thanks,
Adam