This seems like a bug
WARNING: /www/htdocs//plugins/CoreVisualizations/Visualizations/HtmlTable.php(228): Notice - Undefined index: nb_pages_per_search
WARNING: /www/htdocs//core/DataTable/Row.php(188): Warning - Illegal offset type
I did a little trial and error. When I disable the “Site Search Keywords” widget on the dashboard, the error message does not appear. When I turn it on again, the message appears again.