Swathi, do you mean 10.2HF3 ? because 10.3HF2 is not released yet.
Its hard to tell what's causing your issue without any error stack or config details.
The best way to address this is through Support ticket.
My bad, its HF1. Even in the logs there is no information and in DEBUG mode as well, the only exception we see is Request processing exception.
This is a issue with 10.3 version. We got a confirmation from Infa support team that teh asynchronous processing of unmerge request doesn't work with post stage user exit. hence closing the discussion,