When using OWA through NetScaler, users are prompted with logon screen even after entering the correct credentials.
To resolve this issue configure cookie persistence with a timeout of 0 instead of using the SSL session based persistence. When the timeout is set to 0 the cookie would not expire till the browser session is terminated on the user machine.
The issue was caused because the session persistence was getting timed out. This in turn was creating OWA login failure.