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The XML produced by the plug-in can be opened in Excel 2004 or Excel 2008. If you still have one of those hanging around on your system, then open the OO3 Excel XML export, and then save it again from Excel 2004 (or Excel 2008) as an XML export. This can then be opened by Excel 2011.

FWIW, it appears that installing Excel 2011 destroys the ability of Excel 2004 and Excel 2008 to save anything other than XML files. It might be the case that the XML produced by the plug-in isn't actually fully compatible with either Excel 2004 or 2008, but isn't rejected either. Saving a new XML from either 2004 or 2004 makes a file that is sufficiently compatible with 2011, at least for the time being.