Hi Matthias,
1. Make sure that the SQL Server instance is configured to listen for remote connections on port 1433 Also make sure that there are no firewall/router/proxy configurations between MII and WinCC that are blocking the connection.
2. Make sure that the jdbc driver you have is the correct one for the version of SQL Server running with WinCC
3. Once you get the connection, make sure that SQL Server is set up for Mixed Mode authentication or User/Password authentication, and that you have a valid user/password to connect via the MII Data Server.
Another option: WinCC may have an OPC Server enabled. if so, you can install SAP Plant Connectivity either on MII (if a Windows Server), WinCC (if possible), or on a separate Windows Server or Win7 PC, and connect to WinCC via OPC Server, PCo OPC DA, and MII PCoConnector.
Regards, Steve