Nitin,
Run RSECADMIN and click on the ANALYSIS tab. Under that tab click on error logs. Once in that screen click on on configure log recording and enter the user you want to trace-make sure you save. In the previous screen you should then be able to capture information as to what is failing for the user. You'll have to run the report first under the user and then click display. After you're done make sure you remove the ID from recording. This is what i've done in the past when troubleshooting auth issues when users are running queries in the BEX analyzer and portal.
-Wes