In my experience, it's been related to font issues, and usually to auto-activation of fonts. Check that all the fonts your document is calling for are loaded and available. If a font is truly not available, auto-activation keeps sending out calls to activate something it doesn't have access to, which is interfering with the Notes panel. You may also see problems when trying to enter anything in measurement fields, etc.
As for a fix.... Either get the offending font or replace it in the misbehaving documents, and/or turn off auto activation. That last may be a painful fix, especially for your editor, but manually activating fonts is preferable to not being able to work in Notes, or accidentally entering copy into your live text that you didn't mean to. (Especially after you start getting frustrated and start typing some not-nice things.... )
Just out of curiosity, is this K4, Woodwing or something else?