I just succeeded after several hours. I removed the MicroSD card first. I don't know if that was necessary, but nothing else was working at that stage.
Then with the phone connected to the USB port of the computer, I did a Restart of the phone. (Galaxy S2). The computer found a new device (the phone !!!) and installed new drivers !!!
And this after installing new drivers several times from the Kies troubleshooting page !!!
And then it worked !!! I still can't believe it. That's the reason for all the exclamation marks.