During the first AdminUI registration against a policy server the -adminui-setup argument must be used with the XPSRegClient command. The username:password provided needs to match an existing SM administrator's credentials. When registering additional AdminUI instances, the -adminui argument is used with XPSRegClient (omit the -setup). Instead of providing existing credentials this time, the command will be creating a one-time use account in the policy store using the credentials you provide in the XPSRegClient command, so you need to pick a unique value for the username. You must use these newly created credentials the first time you log into the new AdminUI instance to complete the registration.
Use -adminui rather than -adminui-setup with the XPSRegClient command when registering additional AdminUI instances with a policy server.
In a multi-policy server environment, make sure each policy server is using the same encryption key, otherwise it may be impossible to register and/or log into AdminUI instances.