After upgradation of liferay unable to login. Main issue what I figured out is Screen-Name (for any user) is changed when user attempt to login.
e.g. suppose ScreenName in database is "sunil" and when I attempt for login it is changed to "sunil.1".
I got the reply on Linkedin and the problem is due to LDAP integration. To get rid of this error I have added "users.screen.name.always.autogenerate=false" in "portal-ext.properties" file. Ultimately I am able to do the Liferay core from 5.2 to 6.0 version successfully :)