Hi, I had the same issue.
I have no idea what the underlying cause is… but I was able to recover it using JTAG/SWD mode using an Adafruit STLink V2 mini: https://www.adafruit.com/product/2548
Used STLink in Windows to erase all sectors, re-flashed everything including the 0.6.2 bootloader and my particle cloud key… now DFU mode works again.