I have noticed this behavior when I use Dropbox: If I open a document with Document to Go using the "Open In..." function of Dropbox, I cannot save the document back up to Dropbox. I get the same error and if I want to save it, I have to re-name it.
What I figured out was happening was I was getting the error because the file was still open in Dropbox (either because of multi-tasking or because that's the file that's open...honestly not sure why...). Since the file was open in another app/place, any changes I was making could not be saved.
So, what I did was I learned to either (1) remove Dropbox from the multi-tasking bar or (2) go back into Dropbox and navigate away from that document (in effect, closing it).
So, maybe it's the same situation for you? Try closing down File Manager or navigate away from the document in File Manager and see what happens.
Worth a shot, yeah? Otherwise, my suggestion to you would be to use Dropbox in the manner I just described here...
Marilyn
P.S. Dropbox is free for 2GB storage ... so there's no money involved in getting/using it.