I never turn up the volume slider to 100. And I have seen that MC, when switching to loudness mode, automatically turns down the volume slider. But maybe I accidentally put the slider to its full position, I am not quite sure. I didn't put any attention to the slider...
This problem occurred after I had added an explicit 'loudness zone'. I then noticed loudness wasn't enabled. And when I switched to loudness mode in my other zones MC didn't enable loudness there as well.
However, today I deleted the zone and loudness is now working fine. I also tested to add an explicit 'loudness zone' and it's working there as well, so everything seems to be ok.
If this problem occurs again, I will for sure take a more close look at the volume slider... Thanks for informing me!