I realize this thread's a couple of months old now, but I think I have a clue as to what's going on here. There have been reports on the Rockbox forums about similar symptoms to what you're describing.
That said, here's what I think is happening: the storage chip inside your device may have bitten the dust. I believe the same thing happened to Metaphys, the individual responsible for starting the Fuze+ port or Rockbox. He sent it to Pamaury, the developer who's managed to do more for the Fuze+ than anyone else. When Pamaury open the device and poked around, he found that the chip was indeed damaged, and subsequently announced plans to (eventually) update the bootloader to allow booting Rockbox from a memory card.
Of course, if PyschoTR2, or anyone else who has this problem, wants to try formatting, it couldn't hurt. Good luck, peoples!