Hi Sunil
I've researched a bit this error and I have seen one use case where, when doing su, the ulimit that the user it was switching to was the same as the user who was trying to elevate the privileges, and that was not high enough for the privileged user you were trying to become. If you log in directly as that user and you run ulimit -a, and also as the user who is trying to run the sesu, do you get similar results ? In particular it is interesting to check the nproc limit.
Let me know if this helps