Export to .idml and open that, then see if it can be saved with a NEW NAME. See Remove minor corruption by exporting
Is this restricted to one file, or does it happen to all old files, all new files or all files, old and new?
I have this same issue. Yes I can export to an IDML, but why should I have to do that every time. Seems very counterproductive. Any way to fix this where is won't happen anymore?
This was fixed, I believe, in an update. Make sure you've installed the latest patch.