Hi,
sorry for the late reply.
So the problem does not occur when I play singleplayer or have no dedicated server at all running. I also did it the other way around: Starting the dedicated server, starting Sacred 2, experiencing minimizes, then shutting down the server and trying to start a game. Though then I still experienced the minimizes.
Once I let the computer run for a while, these symptoms see to disappear. Looks like there is something else steeling the focus of Sacred or causing it to minimize. Though I do not experience this with other programs... still, thanks for the hint. I will investigate this further.