I have this error. I read the FAQ and double checked I have this error message on http://barnabasnagy.net/wp-content/plugins/wp-slimstat/wp-slimstat-js.php and I have "Invalid data format" displayed. However, the warning message does not want to disappear and it seems the plugin is not collecting data correctly. Eg no pageviews from last Friday to this date.
Could you please advise?