Failed to login to Cognos 8.1 report studio

Hello,
We are not able to log on to a newly installed Cognos Dispatcher version 8.1. While logging into Cognos report studio, we get the following error message:
"The function call to 'CAM-AAA-Authenticate' failed. Reason:"
We are not even getting the login page. As a temporary fix, we have resorted to restarting Cognos services at regular intervals.
Can anyone help us to define some approach on this.
Thanks,
Tim
