Hi Usha,
Please refer to the below threads and SAP notes with different cases of this error message, I think one of the below should be your case and resolution mentioned would help you resolve the problem.
http://scn.sap.com/thread/3297918
http://scn.sap.com/thread/3311820
1763599 - Error: "illegal Access to the viewer, please use a valid URL" when viewing WEBI documents On-Demand
License key has expired. update License key and restart SIA solves the issue
1800752 - 'Error running or editing webi reports on BI 4.0 SP5 Illegal Access to the Viewer, please use a valid url
Restarting tomcat and then the SIA (CMS's and webi servers) resolved the issue
1807343 - Error: "Illegal access to the viewer, please use a valid url." if opening Web Intelligence documents in the BI LaunchPad
Modify the Request Port to an unused port for those servers solved the issue
1740500 - Error "Illegal access to the viewer. Please use a valid URL" or "An error occurred while the applet was initializing" when trying to view or edit a Webi Report
Uninstall and Reinstall of BI platform resolves the issue
1497578 - Illegal Access to the Viewer, please use a valid url.
Restart the Tomcat server.
1757024 - Error message "Illegal access to the viewer" Shows up intermittently when viewing Web Intelligence Reports in Infoview
Thanks
Mallik