Just this morning, I've suddenly become unable to overwrite an active document in InDesign CC 2014. Every save attempt produces the error message "Cannot save the document" (it may be open elsewhere, you can save as, etc.) - but randomly, I'll suddenly be able to save (once), before having the same issue again.
I'm running the latest version of CC on Windows 7. I've trashed my prefs/defaults, exported to IDML and reopened/saved, and restarted my machine. Nothing has made a difference.