Hi Joachim,
thank you! That seems to have solved the issue!
I wasn't aware, that Taylor-Hood elements have inf-sup stability problems and thought that besides their relative simplicity, the inf-sup stability of TH was one the major advantage of these elements. Also the meshes from Netgen have always looked nice and regular to me, which makes the lack of stability here even more surprising to me.
Apart from my belief that Taylor-Hood would be inf-sup stable, one of my main motivations to use Taylor-Hood was to try and keep things simple at this point. But I am aware that HDG would bring some advantages!
Best wishes,
Henry