Sitecore requires ASP.NET Session State to be enabled when using the Page Editor editing interface for editing items. JavaScript errors or other unexpected behavior may be experienced when using the Page Editor interface to edit an item that uses a layout file for which ASP.NET session state is disabled (for example, a layout has an EnableSessionState="false" parameter on the Page directive).
Enable ASP.NET Session State on all layouts that are used on pages that should be editable in the Page Editor.