Good point, @Dylan.
Here’s what I tested:
Made the MacBook act as a core (connected to network via WiFi).
Playing a radio stream simultaneously to the MacBook, on an iPhone (using the MacBook as core) and to a wired PC works nicely!
I then changed the core again (to my wired server).
Same issues as described above. Then I updated the LAN drivers and voila, everything seems to work fine. Currently streaming radio to an iPhone, PC and playing DSD128 over Wifi to the MacBook (downsampled) without hiccups!
So it seems updating the LAN drivers on the server fixed not only the radio streaming issues but also the issue I described in Audio break ups over WiFi.
Have not thought about updating the LAN drivers, since it is a pretty fresh set up system, so it was my impression that Windows will automatically fetch the latest drivers.
Fingers crossed!
Thanks all for the support!
Edit: Seems like I was happy to soon… dropouts still happen, no idea what to change, never had these issues before Roon. When I find the time, I will try to check if the dropouts are related to the distance/obstacles to my router. However, listening to a radio stream on an iphone 2m away from the Router also resulted in breakups.