Excel Download Not working in Server/Site but working in Local System

We have a site, in that we have a option to download excel report, recently we have migrated(using VS 2010) this from .net 2.0 to 4.0 and the hosted server also changed. server same as previous server which is 2003 and MS Excel installed in it. But the same code working fine in my local system(XP, VS-2010). when i click the button to download excel, i m getting the following error.
 
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
 
I follwed the instruction here http://forums.asp.net/p/1303594/3359738.aspx#3359738. after that it worked fine. But again the same error occurs. We have used Interope.Excel dll in project.
 
Please Tell me the solution if u know. Thanks. .