Clicking on header column of GridClientSelectColumn grid row checkboxes remains unchecked. The grid rows becomes selected (style changed) though.
The issue appears only in IE 11, Chrome, Firefox and IE 10 work as expected.
The issue can be reproduced om Telerik's site:
http://demos.telerik.com/aspnet-ajax/grid/examples/functionality/selecting/row-selection/defaultcs.aspx

It seems not to be related to Compabitibiity View.

With the current implementation if the RenderMode is set to Auto in the web.config file it is later automatically changed to Leightweight instead of Mobile.

Currently, the RadGrid exports files to Excel using the legacy .xls format/extension.  However, all Office documents starting with Office 2007 and later support the .xlsx format.  Therefore, the RadGrid should support the ability to export data to either the legacy .xls format or the newer .xlsx format through attribute settings in the RadGrid control.

If an image has a bracket in its path it cannot be exported. In RadEditor a response is not sent back to the browser, RadGrid will throw an exception to notify about a missing path. A workaround for the time being is to avoid having special symbols in images that need to be exported.

Update the Http handler shown in the following forum thread so that it can be used with the latest version of Telerik UI for ASP.NET AJAX:
http://www.telerik.com/community/forums/aspnet-ajax/editor/radeditor-exporttopdf-in-httphandler.aspx#2089976