Is it possible that you have been editing using different versions of Word? Other people have had similar problems when switching between versions.
The probably cause is an XML "tag" error.
The person you need to join this discussion is a volunteer named "Jeeped". He is the "local expert" on XML errors.
If you are familiar with XML/HTML then you could try taking a look yourself at the native XML to see if you can recognize where the XML tags are not matching properly. Make a copy of the file and rename it by adding a ".zip" file extension.
Quick learning about “X” file structures - XML – Editing XML
Pt1: Breaking Into Your Office Open XML Format Documents (2007)
http://msevents.microsoft.com/CUI/WebCastEventDetails.aspx?culture=en-US&EventID=1032319980&CountryCode=US
But what is different about these new formats, and what can the Open XML Formats do for advanced Microsoft Office users? Join this session for a guided tour through the XML behind your documents, along with an introduction to editing the XML directly so you
can get more from your documents than ever before. Learn about the files that make up a Microsoft Office Word 2007, Excel 2007, or PowerPoint 2007 document and get timesaving tips and troubleshooting tricks for editing, sharing, and reusing content without
even opening Word, Excel, or PowerPoint.
Note: This is the first in a series of three sessions for advanced users who want to learn to customize 2007 Office release documents. The second session in this series introduces Microsoft Visual Basic for Applications (VBA). The third session brings concepts
covered in the first two webcasts together. We show you the basics of customizing the Ribbon, including how to add your own VBA macros to built-in or custom Ribbon tabs.
This last DOCX Unspecified error had to do with the
<m:num> XML tag and I’ve had a few of those lately. Often, the <m:num></m:num> pair contains absolutely no content and can be removed without any damage to the DOCX at all. In this case, it was wrapping the letters BH which were the numerator
in a formula. The malformed section looked like this,
<m:num>
<m:r>
<w:rpr>
<w:rFonts w:ascii …. “Cambria math”/>
</w:rpr>
<m:t>
BH
</m:t>
</m:r>
<m:ctrlPr>
<w:rpr>
<w:rFontgs w:ascii*”Cambria math” … “cambria math”/>
<w:1/>
<w:rpr>
<m:ctrlPr>
</m:num>
Now that XML code sequence passes syntax muster by Word still chokes on it. This can be proven by extracting the
word\document.xml and opening it in Excel (or some other XML editor).
I first tried to delete the lower <m:ctrlPr></m:ctrlPr> section as it seemed empty of any actual contribution to the content of the DOCX but that still did not allow Word to open the document. Failing that, I noted other
surrounding text so I could locate the problem area within the document at a later time (e.g. within
Word) and snipped out the entire the <m:num></m:num> block. This allowed Word to open the DOCX.
I searched for the surrounding content I had noted previously and recorded the page number. I zoomed to
10% and took a screenshot of the first four pages and wrote my reply.
Here is another cause for this error:
<snip>
here’s what caused my problem: Where you found the error, I had a checkbox (legacy form field) inside of a plain text content control field. It didn’t cause any problems when left alone, but when I altered only that field (i.e.,
checked the box), saved the doc, and reopened it, presto, there was the error. I guess Word doesn’t like form fields inside of form fields...which is maybe an obvious thing... I’d done it based on a suggestion I found online, because putting the checkbox
inside the plain text content control field allowed the user to actually tab to the checkbox -- Word won’t tab to it otherwise (apparently due to the mix of legacy fields and 2007 fields). Oh well, I guess I can live without that :-) I took the checkbox
out of the plain text field and now the form works fine.
</snip>
XML Notepad 2007
https://www.microsoft.com/download/en/details.aspx?displaylang=en&id=7973
Free MS XML editor