Hello Manmay,
Below is a link on how to properly insert a new email contact into your RoundCube address book. Managing your Address Book in RoundCube
I have tested this on the server with no issue. If it still is giving you problems, we can take a look at your particular account. To do so, please email the email address in question and its password to [email protected] (feel free to change the password to a temporary one if you desire) Also, please comment here if/when you do as we do actively monitor that email address. I will be happy to check into the issue.