Crystal Reports Cannot view report in Crystal Enterprise XI 4.0

Message

I got a login to our newly installed Enterprise XI 4.0 and was told to get my reports running. No further info, so I sit and have a long list of question marks, first of all, what seems to be the connection to the database. 
In our old Enterprise CMC (Version 11.5 from 2004), I just had to update my connection data under Process - Database

Custom Database login Information
Database Select a database driver : Oracle
Server: dbv11
Database: mydb
User: xxx
Password: yyy
Table Prefix: xxx. Use default prefix
Use same Database Logon as when report is run

where I get Server and Database from my Oracle^s tnsnames file:

dbv11=
  (DESCRIPTION=
    (ADDRESS=
      (PROTOCOL=TCP)
      (HOST=dbserver)
      (PORT=1521)
    )
    (CONNECT_DATA=
      (SERVICE_NAME=mydb)
    )
  )

In Enterprise XI 4.0 I set the same parameters under the database configuration option, and get this message when trying to view the reports:

The argument has an 
invalid value 0

As the error message doesn't even mention which argument has the invalid value I can't imagine where I did a mistake.

I checked if the Oracle client was correctly installed, and can ping the database server, tnsping the database and connect with sqlplus to the database ...