Error:  Data structure returned is not convertible in the requested format. Try to call this method with the parameters '&format=original&serialize=1'; you will get the original php data structure serialized. The data structure looks like this: 
 $data = array (
  'date' => 'Sat, May 18',
  'previousPages' => 
  Piwik\DataTable::__set_state(array(
     'rows' => 
    array (
    ),
     'currentId' => 1,
     'depthLevel' => 0,
     'indexNotUpToDate' => true,
     'rebuildIndexContinuously' => false,
     'tableSortedBy' => false,
     'queuedFilters' => 
    array (
    ),
     'disabledFilters' => 
    array (
    ),
     'rowsCountBeforeLimitFilter' => 0,
     'enableRecursiveSort' => false,
     'enableRecursiveFilters' => false,
     'rowsIndexByLabel' => 
    array (
    ),
     'summaryRow' => NULL,
     'totalsRow' => NULL,
     'metadata' => 
    array (
    ),
     'maximumAllowedRows' => 0,
  )),
  'previousSiteSearches' => 
  Piwik\DataTable::__set_state(array(
     'rows' => 
    array (
    ),
     'currentId' => 2,
     'depthLevel' => 0,
     'indexNotUpToDate' => true,
     'rebuildIndexContinuously' => false,
     'tableSortedBy' => false,
     'queuedFilters' => 
    array (
    ),
     'disabledFilters' => 
    array (
    ),
     'rowsCountBeforeLimitFilter' => 0,
     'enableRecursiveSort' => false,
     'enableRecursiveFilters' => false,
     'rowsIndexByLabel' => 
    array (
    ),
     'summaryRow' => NULL,
     'totalsRow' => NULL,
     'metadata' => 
    array (
    ),
     'maximumAllowedRows' => 0,
  )),
  'pageMetrics' => 
  array (
    'loops' => 47040,
    'pageviews' => 47040,
    'entries' => 0,
    'exits' => 0,
  ),
  'followingPages' => 
  Piwik\DataTable::__set_state(array(
     'rows' => 
    array (
    ),
     'currentId' => 3,
     'depthLevel' => 0,
     'indexNotUpToDate' => true,
     'rebuildIndexContinuously' => false,
     'tableSortedBy' => false,
     'queuedFilters' => 
    array (
    ),
     'disabledFilters' => 
    array (
    ),
     'rowsCountBeforeLimitFilter' => 0,
     'enableRecursiveSort' => false,
     'enableRecursiveFilters' => false,
     'rowsIndexByLabel' => 
    array (
    ),
     'summaryRow' => NULL,
     'totalsRow' => NULL,
     'metadata' => 
    array (
    ),
     'maximumAllowedRows' => 0,
  )),
  'followingSiteSearches' => 
  Piwik\DataTable::__set_state(array(
     'rows' => 
    array (
    ),
     'currentId' => 4,
     'depthLevel' => 0,
     'indexNotUpToDate' => true,
     'rebuildIndexContinuously' => false,
     'tableSortedBy' => false,
     'queuedFilters' => 
    array (
    ),
     'disabledFilters' => 
    array (
    ),
     'rowsCountBeforeLimitFilter' => 0,
     'enableRecursiveSort' => false,
     'enableRecursiveFilters' => false,
     'rowsIndexByLabel' => 
    array (
    ),
     'summaryRow' => NULL,
     'totalsRow' => NULL,
     'metadata' => 
    array (
    ),
     'maximumAllowedRows' => 0,
  )),
  'outlinks' => 
  Piwik\DataTable::__set_state(array(
     'rows' => 
    array (
    ),
     'currentId' => 5,
     'depthLevel' => 0,
     'indexNotUpToDate' => true,
     'rebuildIndexContinuously' => false,
     'tableSortedBy' => false,
     'queuedFilters' => 
    array (
    ),
     'disabledFilters' => 
    array (
    ),
     'rowsCountBeforeLimitFilter' => 0,
     'enableRecursiveSort' => false,
     'enableRecursiveFilters' => false,
     'rowsIndexByLabel' => 
    array (
    ),
     'summaryRow' => NULL,
     'totalsRow' => NULL,
     'metadata' => 
    array (
    ),
     'maximumAllowedRows' => 0,
  )),
  'downloads' => 
  Piwik\DataTable::__set_state(array(
     'rows' => 
    array (
    ),
     'currentId' => 6,
     'depthLevel' => 0,
     'indexNotUpToDate' => true,
     'rebuildIndexContinuously' => false,
     'tableSortedBy' => false,
     'queuedFilters' => 
    array (
    ),
     'disabledFilters' => 
    array (
    ),
     'rowsCountBeforeLimitFilter' => 0,
     'enableRecursiveSort' => false,
     'enableRecursiveFilters' => false,
     'rowsIndexByLabel' => 
    array (
    ),
     'summaryRow' => NULL,
     'totalsRow' => NULL,
     'metadata' => 
    array (
    ),
     'maximumAllowedRows' => 0,
  )),
  'referrers' => 
  array (
    0 => 
    array (
      'label' => 'Direct entries',
      'shortName' => 'Direct Entry',
      'visits' => 0,
    ),
  ),
);