IE8 "Unspecified Error ckeditor.js, line 63 character 296" on CKEditor 4 load

I updated from CKEditor 3 to 4, updated the custom config, plugins etc, and everything seems to work ok, except that sometimes when the editor instance loads I get this error "Unspecified Error ckeditor.js, line 63 character 296".

This seems to happen more if I have formatted text using the new editor version.  What I mean is that I can load existing text, Bold a line or change the font, save it, but the next time I try to load it, I get this error.  It's very odd.

The section of ckeditor.js that is highlighted by IE8 debugger is:

return new CKEDITOR.dom.element(this.$.activeElement)

This is sporadic, doesn't happen every time, and never used to happen with CKEditor3.

Attached is:

(a) The code on the page that activates the editor

(b) The editor content that is being loaded

(c) The custom config.

Hopefully this means something to someone. I have Googled extensively and been unable to find the same error highlighted anywhere else.
 

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Edited to add:

I should note that I only seem to get this problem in IE8, not in Firefox or Chrome.

Just now I loaded and saved content fine, went to reload the page and got this error.  In case it's relevant, it seems to not necessarily be the first time I load a page, but on subsequent opens, which is odd. The only error I am getting is with the editor loading, and the result is the page loading but no editor content.