Thanks for your reply. The player/HDD is completely healthy, I didn't notice any other glitch.
It never locked up during normal usage. All the files are mp3, error free
and properly tagged. The bitrate is varying.

I didn't reformat the drive after upgrading the firmware. But let me put it this way:
I was unable to reproduce the same lockup using RMM over USB.
The problem feels like a race condition/locking issue when using Ethernet.