This was an annoying issue to solve – I was creating spreadsheets in python using openpyxl. Opening them in desktop Excel (on macOS, but I’ve seen this error on Windows too) resulted in an alert: “We found a problem with some content in ’file.xlsx’. Do you want us to try to recover as much as we can? If you trust the source of this workbook, click Yes.”. Excel was able to repair the file fine, but it was annoying. Looks like this might have been an issue openpyxl version 3.1.2 – installing 3.0.10 in a venv solved the issue.