Strange behavior of Excel 2003

Since about a couple of months, the Excel from Office 2003 Pro started
misbehaving. When I try to save a particular document, it says that a sharing
violation occurred and offers to save it under a different name. Apparently the
sharing violation is with itself, since nobody else edits that document. Saving
it under a different name works, until one tries to just save it, when again the
violation happens. It didn't do that before; something changed. Any idea what it
might be and how to make it work normally with this file again?
The circumstances:
- a virtualized XP Professional in VirtualBox
- the issue happens only with this one particular document, other documents save OK
- the issue happens only on a virtual drive; in the original computer it was a
separate physical drive which I copied into a folder on the host and attached as
a logical drive. From the POV of the XP it is still a separate drive with the
same drive letter F: as it was when it was physical. The issue does /not/ happen
when saving the particular document on C: in let's say \My Documents.
Thanks for any ideas you might have.

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