This is a range extender, right? We’ve seen many, many problems caused by range extenders (as discussed here), so I would strongly recommend at least testing with a fully wired configuration.
I see that your NAS and Core are wired, but I’m guessing your network runs off a different router? Or am I misunderstanding how you’re using the Netgear?
Another thing to try would be to move a file to the Mac Mini’s internal drive, like to the Music folder, and to see if that plays back without issue. If that works, we know there’s an issue between the Core and NAS. If that doesn’t work, we know there’s an issue between the Core and the audio device.
Can you try those tests and let me know how it goes @Lars_Johansson – once we get you working we can restart the clock on your free trial too, so you have the full 14 days to try Roon.
Thanks!