Are you also on 3.3g (what build if so)?
Are you able to actually save the documents, and the only problem is that you have temp files being left? Or does word hang, and when you try to open the document again it is corrupted.
Can you try this:
1. in Company Home, create a space called test (or whatever you like)
2. Assign a user a with Coordinator role to that space
3. Login to cifs, create a word document, open and try to do edits.
Report back what happens.
Reason I'm asking is that no matter what role the user has been assigned, even a role with full permissions, the save error is present. And that look likes it is a bit different that your error. But I havent actually tried to edit within the Sites path, only for a test space in Company Home as described above.