Dear Soumya,
We also faced the same issue and checked all relevant notes to this issue but no luck.
Finally we identified as there is a problem in the WEBSEAL policy as no authentication ploicy added to the BI system.
Try to add the noauthentication policy with the help of your portal administrator and the issue will be solve.Fore more information follow the below link.
Logon Pop up while exporting data to Excel
Best Regards,
Sai