Hello again,
lucky you, I’ve figured the solution myself:
The problem is not the driver but the device settings on Windows.
In the taskbar, right click on the little speaker icon and choose Sound Settings. Like shown in my screenshot, go to the properties of your internal speakers or headphones output and then click on the Advanced text (see also screenhot), a new little window will open. In that new window go to the Advanced tab and voila, at the bottom there uncheck the Enable audio enhancement option (see also screenshot on the right).
That’s it, press Apply and the speakers/headphones will be stereo again, even when in non-exclusive mode of the Generic Low Latency Driver.