I finally tested connecting the Mu-so directly via Ethernet to the router and via Wi-Fi (disconnecting the Wi-Fi repeater, where the Mu-so is usually connected to). I saw it connecting via 2.4 GHz and again it sometimes worked, sometimes not.
I did a reset (short press via pinhole during power-on) and a deep reset (following the procedure here: Hoping Naim mu so isn't dead - #15 by Tucker ), but all without success. The same error keeps reappearing. I assume the only possibility left is a HW issue.
Stupid of me not to question this earlier, as I had this issue since the beginning but put it to be a connection issue…
I’ll contact the Naim support, but as I don’t live in the UK, I don’t have high hopes to get a solution. If you have any other possible ideas, I’m happy to hear them! I’ll post an update as soon as I get some news by the Naim support.