The above issue is resolved by correcting the load balencer setting in Access Manager setup. We hit the next issue as below:
when a protected resource is accessed first time, OAM_node1 is generating the "request_id" and the login form is thrown to user.
Now user keys in username and password, then submits. This time, based on round robin algorithm, the http request reaches OAM_node2 where it complains that it does not know the "request_id".