ACS 5.1 is failing to authenticate tacacs authentication to the ASA firewall, getting
That's what I suspected. You will have to deregister the secondary ACS from the Primary. Configure the appropriate Secondary ACS clock and timezone to match the AD Domain Controllers time. Both the clock change and the timezone change will restart the secondary ACS services for the changes to take effect.
After the appropriate time has been configured we should "Test Connection" against AD from the ACS GUI on the secondary. As soon as it succeds we can proceed and save changes and also register the secondary back to the primary.
This should address the issue.