In short answer, XDocReport doesn't support odt->docx converter.
You use report.process
which means that there is no conversion (docx template->docx report, odt template->odt report). In your sample, the generated report is an odt (even if you set filename with docx extension), that's why OpenOffice is enable to open it although MS Word cannot open it.
If you wish to convert the report to other format like html, pdf you must use report.convert
but in your case you need a docx->odt converter that XDocReport doesn't provide. But as XDocReport is modular you could develop your own docx->odt converter and plug it with XDocReport.