Hi Bala
Run a ST01 trace on the action to see if authorisation is a cause. Test it in development as it's unlikely you will have multiple application servers. Note that the trace in the plug-in will be for the FF Id account and not the person trying to use FF. I cannot remember 5.3 off the top and don't have a system to run the trace in. As a guide you could always look at the /VIRSA roles to see what they recommend.
What use type is the FF Id and what is maintained for the RFC connection? Does this RFC connection get referenced in your FF configuration?
To confirm, as you are on 5.3 does this mean the FF is being used locally (that is the FF cockpit login is on the ECC)?
Regards
Colleen