Thanks for template file and sample code.
After an initial test, I observed the issue as you mentioned. I found the exception “Object reference not set to an instance of an object” when opening the re-saved XLSX file via Aspose.Cells APIs. I used the following sample code to first convert your template XLS file to XLSX and then open it via Aspose.Cells APIs:
Workbook workbook = new Workbook(@“e:\test2\Example.xls”);
workbook = new Workbook(@“e:\test2\outExample1.xlsx”); //error on this line: Object reference not set to an instance of an object.
I have logged a ticket with an id “CELLSNET-43810” for your issue. We will look into it soon.
Once we have any update on it, we will let you know here.