Hi Uruj,
I would suggest you to perform the following steps:
a. login to Clarity application as an admin user
b. modify URL to: http://<svr>/niku/nu#action:security.caches
c. click Caches tab, click FLUSH ALL button
d. click Groups tab, click FLUSH ALL button
e. logout
f. login and check if problem persists.
g. if problem persists, restart all APPs
Note: Clear the Caches when the user load is minimum, Downtime is not needed though. As the activity may close all the active sessions.
Thanks,
Abhisek Dhar