Created Unassigned: xlApp.Workbooks.Open throwing com exception [15233]

Hi,

What are the possible causes of the xlApp.Workbooks.Open(<filename>) throwing a System.Runtime.InteropServices.COMException? I tried to check the stack trace and I can't find a cause other than the InnerException being a NullPointerException

Interop Error is: HRESULT: 0x800A03EC
Error Code: -2146827284

Basically after the Open statement, the next statements before the process is completed is as follows:

//other page setting details

xlWorkbook.Save();
xlWorkbook.SaveAs(strFileName.Replace(".xlsx", ".xlsb"), _Excel.XlFileFormat.xlExcel12);
File.Delete(strFileName);

The file that it is attempting to save can already exist or can be a new file.

This is based on the download source code (epplus-59920b4b5f9f.zip)