CKEditor fails to load when used by Panels due to a missing basepath setting

When adding custom content into a panel via the Panels module, CKEditor fails to load. See the attached screenshot. The path to the language file is

/node/3/lang/da.js?t=C6HH5UF

, which suggests that the CKEDITOR_BASEPATH variable is not set.

Steps to reproduce:

• On a clean installation, install Panels and Wysiwyg with CKEditor in /sites/all/libraries/ckeditor
• Add a HTML text format and make CKEditor be used by that format.
• Create a node of type Panel and click add custom content.

A temporary workaround is to add the following line in the top of ckeditor.js:

window.CKEDITOR_BASEPATH = '/sites/all/libraries/ckeditor/';