BA Configurator fails to login

Hi
We are trying to setup Business Analytics with NAV 5.0 with the Cronus DB on SQL Server 2005. After entering configurtor path db name and db-instance we start the configurator. The first steps initialize, check for sql server and analysis server are sucessfull. The next steps faild on position one Build Internal Representation with the error: Login error for the user su. This user has all rights on the sql server also we synchronized the logins from Navision. What else must be done to solve this security issue?

Thanks for any help.
Jan