Dear Biswajit
I hope you are well and many thanks for using the SAP Discussion Forums 
.
In relation to the scenario that you have described you mentioned the following:
- You have a WDJ application which connects to a back end system to fetch outstanding work items for the user. For some users it is not fetching any items and throwing the following error
- No RFC authorization for function module
Ok firstly in relation to the description that you have outlined above did you attempt to clear the portal caches and restart the portal service?
Kindly reference the following documentation:
Resolve RFC Generation Errors
http://help.sap.com/saphelp_smehp1/helpdata/en/6f/7fed65ffa94a76966cd2ca6fbafeb6/content.htm
Additionally kindly review: SAP Note : 1108662
Kindly update me as per your findings.
I greatly appreciate your time and patience and I look forward to recieving your reply.
Kind Regards
Troy Cronin - Enterprise Portal Support Engineer