So either some thing is happening while switching the url between portal to backend webgui or the configuration of the iview on the portal is not correct. Hope used the same id and same alv report to do the above test .
Thanks for your help. Actually we are not running this sap transaction in portal. Its running in SRS(SAP Retail Store).
http://help.sap.com/saphelp_470/helpdata/en/fb/89ef37516fdb2be10000009b38f842/content.htm
SRS is a browser based solution for SAP Retails stores. It uses ITS. We are calling a SAP Transaction from this SRS.
I have tried to do web debugger but still not getting any pointer to fix it.We have some ALV reports in SRS and we are able to download the data for those ALV reports. The problem is with this transaction. This transaction saves a file on local machine.
One more thing - if we run the same transction in Chrome browser then we are not getting this I/O Error.
(The problem is Not solved yet)