Somehow my audio output gets mapped to both channels, even if only one (stereo) channel is active (that is, only one of the two channels in the mixer of the Master bus is lighting up).
I've checked the channel mapping in the the "Preferences / Speaker Setup. It doesn't make a difference if I "Use System Settings" or manual setup.
By testing with another program I found out that this is actually a Windows problem. It happens with the built in speakers, an external (USB) DAC and when connecting headphones through Bluetooth.
Windows seems to pan both the left and right channels to the centre, so even when I set the balance (in the audio program) to "left", the output plays through both channels.
I still couldn't figure out what is causing the problem. Maybe there is someone out there on Windows 2004 who could check if this is my "personal" problem or not?
After hours of googling, I found the problem: there is a "hidden" option in W10 to turn all audio "mono". It is not part of the audio settings, but of the accessibility settings. I had never changed it, but it was activated. Disabling it resolved the problem.