New Post: "unreadable content" error when opening Excel file created via ClosedXML

You may need to set the cells DataType. This is the only thing that worked in my case.

ex..

ApprovedCoursesSheet.Cell(Row, 1).DataType = XLCellValues.Text
ApprovedCoursesSheet.Cell(Row, 1).SetValue(Of Int32)(c.APPL_COURSE_ID)
ApprovedCoursesSheet.Cell(Row, 2).DataType = XLCellValues.Text
ApprovedCoursesSheet.Cell(Row, 2).SetValue(Of String)(c.APPL_COURSE_NAME)