Somehow the network installation of WP-Piwik doesn't point exactly on every blog. When selecting a specific date in the overview-list the Plugin shows the webstatistics of an other blog. This goes whole through my entire Multisite.
Though the network capabilities are still experimental and I don't know yet, if this behaviour is originated on my own, I have to say "it's broken". Wish, there would be finer rating system within the WordPress Plugin Directory, because in all other cases WP-Piwik works fine.