I set it to major because it took me hours to debug.

I finally found the problem. It's in ckeditor.utils.js, there's a part "Support for imageField" which was added because of #1286192: [D6] Fix for CKEditor support to Imagefield module textareas (#1643360: Content ckeditor deleted when removing a file upload might be related too).
There is a jQuery selector that should get the image's title field if I'm right, but sometimes there is no title field for the image field (it's configurable). In my case, the body field of my content get selected instead. This generates a "TypeError: x is undefined" error, or in uncompressed CKEditor "toolbar is undefined".

Looks like the bug can be easily fixed by adding .imagefield-text in the selector, like this:

if ($('.form-item textarea.imagefield-text', $(context)).length == 1) {
      var url = document.location.pathname;
      if (url.indexOf('/') == 0 ) url =  url.substr(1);
      var path = Drupal.settings.basePath;
      var imagefield_id = $('textarea.imagefield-text', $(context)).attr('id');

It's interesting to notice that the bug does not happen for certain roles (in my case for user 1), because the log textarea field is present so that $('.form-item textarea', $(context)).length equals 2.