Gimp XCF export doesn't work on Windows

I've tweaked my system and Gimp installation so that XCF export shows in the menu (Gimp path added to the PATH env, and gimp-2.6.exe duplicated and renamed gimp.exe) but using it returns a script error message:

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Traceback (most recent call last):
  File "gimp_xcf.py", line 189, in <module>
    e.affect()
  File "C:\Devel\Inkscape\bzr\inkscape\share\extensions\inkex.py", line 215, in affect
    self.effect()
  File "gimp_xcf.py", line 183, in effect
    x = open(xcf, 'r')
IOError: [Errno 2] No such file or directory: 'c:\\docume~1\\nico\\locals~1\\temp\\tmp8s3dqw\\test.xcf'
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The temp folder contains the extracted layers (png files) and a junk file created by Gimp (attached).
According to the junk file messages, the extension fails because Gimp (or the scheme interpreter) can't find the layer files, and thus can't create the XCF file.