This error sometimes indicates that EPPlus is unable to properly read the Excel document, and the issue could be due to a corrupt Excel file, rather than a permission or disk issue as the error might misleadingly suggest.

After encountering this issue myself, here’s what I did to resolve it:

1. Open the Excel file in Microsoft Excel.

2. Copy all the contents of the sheet

3. Create a new blank Excel workbook.

4. Paste values only into the new workbook (Right-click > Paste Special > Values).

5. Save the new workbook and upload (the new one) again.

I was only able to detect this because I tried another library SlapKit.Excel which displayed a more user-friendly error message.