Authentication failures with CommVault generally will end up being one of two issues:
1. Communication problem between the CommServe and the Client (firewall/port issue)
2. The name in the registry does not match the name in the Database. This is the least likely unless the client name was recently changed.
If no firewall involved, try
to ping each server (CS to Client and Client to CS)
telnet to each other over ports 8400 + 8402 (8400 will give blank screen if successful and 8402 will put two lines of "jibberish"