Had really annoying problem yesterday. Was busy setting up Role-Based TACACS access on Cisco ACS and happily configuring the NDG (Network Device groups), the Command Authorization Sets etc.
Started testing and kept on getting “% Authentication failed” on the CLI.
At first you realize you must have made a typo or forgot to do something. Double check the config, the ACS setup and confirm the passwords are correct with no Null Spaces. But still no luck.