Please try factory resetting the device by holding the reset button until all signal LEDs light up. If this does not fix the issue, you could try re-flashing the firmware using the bootloader procedure described here.
You will most likely need to install the legacy firmware first, and then from the WebUI of the legacy firmware update to the latest firmware.
Additionally, if that does not help, I will ask you to log into the CLI/SSH of the device and run this command:
hexdump -C /dev/mtd1
and send me the output via a private message.