I'm not sure if this could be your issue, but anyway take a look, please.
In my case, the problem was online edition on Windows 7 and Office v. 2010.
A possible solution, which has worked with Alfresco 5.0d in Windows 7:
1.- Open "regedit"
2.- Look for the entry "HKEY_CURRENT_USER"> "Software"> "Microsoft"> "Office"
3.- Depending on the version of Office that we have installed, the folder may vary; in our case: "14.0"> "Common"> "Internet"
4.- Add a new entry to the registry named: "BasicAuthLevel" and set the value "2"
5.- Try again to edit online the document that we have in Alfresco and, if everything its ok, this will open correctly for editing after requesting the credentials
6.- We will modify the document and save it.
7.- Close Microsoft Word and return to Alfresco, where - from the preview - we will verify that the document has been modified
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