Two Airplay devices, only one working


I have a weird airplay issue since I swapped my RoonServer from my iMac to an Intel NUC running Ubuntu Linux (ubuntu-16.04.1). In my setup I use two (2) Apple Airports. This has worked fine on the iMac but not on the Intel box.
I still can choose both Airports and in the client screen it seems as if the audio is playing to the Airports. No errors whatsoever. But only one outputs audio. I have tried another Linux too, be with the same result.
Maybe it has to do with the Intel NUC (NUC6i5SYH with Intel Core i5-6260U).
Has anyone encountered this weird behavior?