Possible causes for this error message:
Problem: The host name is invalid or your security server cannot find it.
Solution: Verify your realm configuration.
Problem: A connection to the host could not be established.
Solution: Make sure that the host is functioning properly.
Problem: There is a problem with your principal profile.
Solution: Confirm that your default principal profile is correct.
Problem: Double-check the case you used when entering the realm name.
Solution: Typically, it should be uppercase. If you are using a DCE security server, the cell name (DCE equivalent of realm) must be lowercase.