Thanks for considering Aspose.
Well, the error does not seem to have much sense or relate to Aspose.Cells component. The error might relate to the code (on the .asp page) other than Aspose.Cells APIs, please check to sort it out.
Could you simply try the following code with your template file and check if it works fine.
'Create an Excel object
Set xls = CreateObject("Aspose.Cells.Workbook")
'Open a designer file (template)
'Put data into this file
Set sheet = xls.worksheets.item(0)
Set cells = sheet.cells
Set cell = cells.item_3("A1")
cell.PutValue_5 "Hello World!"
'Save the document to the stream
set stream = xls.SaveToStream()
'Specify the document type
Response.ContentType = "application/vnd.ms-excel"
'Specify how the document is sent to the browser.
Response.AddHeader "content-disposition","attachment; filename=MyBook.xls"
'Get data bytes from the stream and send it to the response.
bytes = stream.ToArray()
If you still find some problems, please do post your template file here.