Cyrus uses its own authentication database based on sasl. You actually need to use the cyradm tool and the sasl tools to create users in cyrus, these are not the same users as on your linux box.
Another question, did you actually create user accounts and if so, did you set an initial password?
You could try setting the password for one of the users using passwd <user> while logged in as root. You should also check if the users have homedirs. If not X will probably not be able to start... And check the permissions to the dirs.
I hope this helps you,