Navision & SQL server


Can anyone tell me how Navision accesses SQL server? I have this problem where Navision has to access beside our Navision database also another database which is linked to Navision database. So when i try to access (in Navision) tables from the other DB i get this error message:Server user 'prosen' is not a valid user in database 'otherDB'. This message is displayed until i grant user System Administrator role on SQL server.
If anyone has any ideas how to solve this, please help.

lp prosen