I would take a look at the xml being produced.
First step is to "unzip" the xlsx file after it has been created - before you try and open it with Excel.
You can use any standard "Zip" program such as 7-zip for this.
In the unzipped content, you will look for the file /xl/worksheets/sheet1.xml
Open that with Notepad++ or similar (or an actual xml editor) and check whether it has been truncated, thus corrupting the XML inside.
If the file appears to be valid XML, you could try locating the specific location it reports as a problem (Character 282473463) and see if anything looks "bad".
MM
select geometry::STGeomFromWKB(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