Excel will block some versions of excel worksheets in the Trust Center settings which prevents the spreadsheet from being edited once it is opened. To prevent this from happening you may need to edit the Trust Center settings for certain file types.
Click the Excel File tab, select the Options menu.
In the Options window, click Trust Center > Trust Center Settings.
In the Trust Center window, click File Block Settings, and then clear the "Open" and "Save" check box for the file types that are checked. There should be no checked boxes specifically on the “Excel 97 – 2003 Workbooks and Templates” and “Excel 2 Worksheets”.
To see which settings you need to modify, refer to the screenshots posted below.