[RCU] Roundcube login error

I had installed and configured RoundCube 1.0.4 as an internet User Interface. The problem that I am facing is that whenever I am using ipaddress of the server to login to roundcube there is no problem but whenever I am writing the domain name of my server in the address bar , every time the login page displays but on typing the username, password and submitting it displays the error " Your session is invalid or expired "..

I have raised tickets in Roundcube but could not get much help. Can someone help me on this issue?

Thanks in advance.

Sharmistha B