Daniel,
the reading does not break anything. It's the saving. Whenever XLR8 saves a file, it parses all information on cells, styles, fonts, etc and creates new XML files to then build the new XLSX file.
The underlying error in this case are issues when parsing the font family. We will investigate why this should ever break anything. When you investigate in the error message you provided, you will find that it fails at the closing tag for the "font family" entry.
A workaround that did the trick the for me: Open the original file in Excel and re-apply the "Arial" font to the cells. Save the file. After that the VI did no longer modify the Excel file.
Cheers
Peter
CLD | Alliance Partner | Web data storage and visualization | daq.io