In Office 365, there is a Trust-Center setting where you can define which file types you trust. I have noticed it only recently (as I had to open a very old Excel 4 Workbook), but I cannot tell when it was introduced or if the default settings changed recently.
You reach this settings via
- File->Options
- Trust Center -> Button Trust Center
- File Block Settings
xls-files can contain Macros (and you can't tell from outside if they have or not). Checking "Open" or "Save", Excel prevents that you can open resp. save those files, so in my case, it is allowed to open xls files.
Update Haven't had a close enough look to the linked MS side, sorry for that. So my answer is basically repeating that.
To narrow down the problem: Have your tried to do the same simply with VBA?
Sub testSaveAs()
Const pstrSrcFileName = "C:\Files\Book1.xls"
Const pstrDestFileName = "C:\Files\einfotree Copy of Book1.xls"
Dim Wb As Workbook
Set Wb = Workbooks.Open(pstrSrcFileName)
Wb.SaveAs pstrDestFileName, xlExcel8
Wb.Close False
End Sub
If this doesn't work, problem is on Excel side. If it works but your VB.Net code not, it's somehow related to Office Automation.