Looks like I have the same problem.
I carried on loading up to around 400GB, but when I restarted the iPod I got a Rockbox error message. On reboot, the splash screen appeared, then the Rockbox menu appeared briefly, then the error message again. It was late last night so I left it, intending to return this morning and note down the message.
However the iPod appeared to work okay this morning and I was able to play music. After a short while though I started getting drop-outs while it was playing and when I rebooted the player I couldn't get past the splash screen.
I've now reformatted the SSD and reinstalled Rockbox so will reload it up to 385GB and see if it remains stable.