Couldn't figure this one out for the longest time. Console was showing an error along the lines of "e is undefined".

The problem was that in the summary field, if I ever went into "Source" the editor would crash and keep all of the keys greyed out (unable to edit anything). Also changed done to the field prior weren't being saved. Syntax Highlighter 7.x-1.1 was the one causing problems for me.

I uninstalled Syntax Highlighter and all the problems went away. Using Code Filter (seems light and less buggy anyways).

Comments

Thank you for report this. We will check this. Please be patient.

Version:7.x-1.6» 7.x-1.13

Two years later, I just encountered the same issue.

Using:
Syntaxhighlighter Insert 7.x-1.1
CKEditor 7.x-1.13
CKEditor 4.1.1 Full

Recently upgraded to 7.x-1.13 and 4.1.1 Full and encountered this error. Clicking the syntax highlighter option in CKEditor will launch a blank modal screen that greys out all options. Only way out it to refresh the browser, losing all work. :(