Hi Sunny,
I believe you are calling a transaction from Portal and trying to save / download the excel from there.
If this is the case, directly access the transaction through GUI, and then try to save / download the excel file and check if it there is the same behavior.
Also, try with some other user id as well or probably on a different machine.
Moreover, as this was working till few days back, can you please verify if there has been any change on the server side Portal or R/3.
BR,
Anurag