It was about 24 hours ago (so 10 AM yesterday, locat time). The issue was ARC on Android auto, and ARC directly on the smartphone. No Issue with other apps on Android auto or on the smartphone.
But hey, a new version of the core became available yesterday late afternoon, and low and behold: all is well now (at least on ARC native on smartphone - not in my car at this time). I even tested playing “local music” (not on Qobus but stored on the Core) and that also works like a charm.
Edit: strange this though: when ARC connected to my core, it said that was the first connection to my core. It was not. I had (briefly) used ARC on my phone before.
This thread seems to explain what happened:
But hey, all is well that ends well.