CKSource

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.

Internet Explorer Enhanced Security Configuration blocking CKEditor

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 about:blank address.

Adding about:blank to Internet Explorer Trusted Sites

Additionally, if you are using CKEditor's built-in spell checker, you should also add the SpellChecker web addresses to the Trusted sites.

Internet Explorer Enhanced Security Configuration blocking the spell checker

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.