I debugged this a bit and think it is related to the following code in
application/helpers/admin/statistics_helper.php

being called twice when view stats is called - could it be that the value of the post parameter gets converted twice which messes things up?

            //check for datestamp of given answer
            elseif (substr($pv, 0, 9) == "datestamp")
            {
                //timestamp equals
                $formatdata=getDateFormatData(Yii::app()->session['dateformat']);
                if (substr($pv, -1, 1) == "E" && !empty($_POST[$pv]))
                {
                    $datetimeobj = new Date_Time_Converter($_POST[$pv], $formatdata['phpdate'].' H:i');
                    $_POST[$pv]=$datetimeobj->convert("Y-m-d");
                    $selects[] = Yii::app()->db->quoteColumnName('datestamp')." >= '".$_POST[$pv]." 00:00:00' and ".Yii::app()->db->quoteColumnName('datestamp')." <\

= '".$_POST[$pv]." 23:59:59'";
}