This is a solved problem but I thought I would post in case it helps anyone else.
Yesterday the NDX2 would suddenly not play HiRes tracks either streamed from Qobuz or off a USB. I will not bore you with the time it took to narrow down the problem to it being HiRes files only and definitely being the NDX2 but so it was. A normal reboot did not sort it but a full power off reboot did.
So if your NDX2 suddenly makes HiRes files sound like one of Stockhausen’s finest efforts, try a power off reboot!