Dear Amy,
thanks for your help! I played around a bit more and it looks like it's a microsoft-problem. It's quite easy to reproduce (at least for me with Office 2013. Finally, Microsoft managed to exclude (or hide) the usual info about the version of the software, so I can't tell the specific version, but I should be up to date with it.)
So, put a simple Excel-File and a Word file in the same folder. Open the excel-file and mark 2 cells, copy them. Open your word file and paste the link (Paste -> Pate special ... -> Choose "Paste Link" and as "Microsoft Excel Workbook Object", hit OK [Wording might be incorrect, I translated it from the German version]). If you press Alt+F9 in Word, you will be displayed the "code" behind the link instead of the value. It should show something like
LINK Excel.Sheet.12 "C:\\tmp\\Test\\Excel-Test.xlsx" "Tabelle1!Z1S1:Z2S1" \a \p
Now, save the file. Make sure, you are still in "display-code-mode". Close it. Open it. You'll get a message that Word want's to refresh the links. In the Word file, one can already see the loaded content. And see that it changed to:
LINK Excel.Sheet.12 "C:\\tmp\\Test\\Excel-Test.xlsx!Tabelle1!Z1S1:Z2S1" "" \a \p
Obviously, when Word saves the document, it breaks the LINK-code. Hit whatever you want about the refresh message. After that, the parser of Word is able to restore the broken code again, so you won't realise what happend, when you don't see the code of the LINK.
And this is probably also the reason, why your parser can't load the docx. I also tried open OpenXML-SDK, it can open the File, but it's still unable to correctly interpret the broken LINK, so I am unable to work with it.
I attached my Office-files, removing the Path, so Word can actually find the corresponding Excel-file in the same folder.
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