When you are running the Windows Server system and the Internet Explorer browser in Enhanced Security Configuration mode, note that CKEditor for SharePoint will not work unless
about:blank is added to the Trusted sites zone.
To overcome this problem you should add
about:blank to the Trusted sites zone. Open the Internet Explorer Tools menu and go to Internet options. Move to the Security tab and select the Trusted sites icon. Click the Sites button and add the
Additionally, if you are using CKEditor's built-in spell checker, you should also add the SpellChecker web addresses to the Trusted sites.
This limitation is not related to CKEditor but is a consequence of using Internet Explorer Enhanced Security Configuration on the Windows Server system. Client browsers using CKEditor for SharePoint installed on Windows Server will not be affected.