System.Runtime.InteropServices.COMException

Error message:
Excel cannot open the file '[NAME_OF_FILE].xlsx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

Description:
System.Runtime.InteropServices.COMException (0x800A03EC): Excel cannot open the file '[NAME_OF_FILE].xlsx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

This error occurs due to Microsoft Windows Server officially not supporting the Microsoft Office automation services, which we use to create the Word and Excel files. You should not encounter any such issues in a Windows client installation (Windows 7, 8, or 10 for example).

For more information from Microsoft please see the following link:

https://support.microsoft.com/en-us/help/257757/considerations-for-server-side-automation-of-office