Thanks Francois, unfortunately I've done both of those things, the file is unlocked and it's "read and write".
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Did you try with the terminal?
Open a terminal and type the following command:
chmod ugo+rw Then type a space, then drag your problematic file on the terminal window and presse Return.
This should change the file permisssions.
If if doesn't work, then you may have more serious problems... Trying to copy or duplicating files, as Sam above suggested, might help to troubleshoot your issue. Using copy/paste into a new document should, of course, do the trick.
Let me know if it helped...