It seems that FreeSpire.XLS doesn't support latest Excel 2016 files.
I wrote simple code:
Workbook wb = new Workbook();
wb.LoadFromFile(@"E:\Sample.xlsx");
wb.SaveToFile(@"E:\Sample2.xlsx");
Sample.xlsx was created 6 month ago and code works fine. When I opened and saved it with the latest Excel 2016 i got error message that the file Sample2.xlsx is corrupted and couldn't be repaired.