I’m having a similar issue.
One SC5000 updated fine via USB to PC.
The other is freaking out. It freezes after turning on at random times (ie. sometimes at the loading splash showing v 1.2.1, sometimes I get to the source page, sometimes I can load a track - but can never get it far enough to play). It has to be hard-booted each time by holding the power in for 5 seconds.
Have tried re-updating it via USB several times - the process works fine but the result is the same. Cannot update via USB now because I think it does not consider a v1.2.1 image on the USB to be an update to the already running v1.2.1 on the player.