Hi Mohan
I hope you are well and many thanks for using the SAP Discussion Forums 
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In relation to the scenario that you have described you mentioned the following:
- You are on Portal 7.4 and ECC EHP7, basic setting for UWL config under System Admin in Portal has been done, apart from that no other changes been made in SWFVISU and to the properties of PCD objects in Portal.
- Working: Leave request respective to the Manager are appearing under UWL
- Not Working: Clicking on any link from UWL taking us to the same screen for leave requests.
Now based upon the extract you provided from the HTTP Watch Trace have you captured a default trace error log file yourself to ensure there are no accompanied underlying error exceptions?
What is interesting here is that you mentioned no other changes apart from the upgrade have been performed. So what we should have here is a decision task that then launches either a Web Dynpro ABAP or a SAPGUI or in turn raises an event.
Kindly capture a trace and let me know if there are any underlying exceptions appearing.
Kindly update me as per your findings.
Kind Regards
Troy Cronin - Enterprise Portal Support Engineer