Is it possible to let the engine play against other (Chessbase) engines at the Chessbase online server?

The Chessbase online server uses a proprietary protocol which is not supported by the ChessPartner GUI. You can play on line on servers such as FICS or ICS. If you want to run the CT15 engine in the Chessbase GUI that is not directly possible. We do know some customers still have accomplished this by using auto232 or with a special program and ERT. See here: