All LEDs blinking blue after factory reset

We have had some problems with one of our routers - ssh daemon and lighttpd randomly stopped working. We replaced it with a spare one and tried to reset it to factory default by keeping the reset button pressed and releasing it when the 4th LED was lit up. After some seconds and LED blinking all LEDs started blinking blue. After reset it wakes up with TurrisOS 5.3.0, so it doesn’t seem that the factory reset was successful.
What can be the reason for such a behavour? How can I successfully reflash the router?

Try different usb sticks as some have tendency to simply not work also try different usb ports. While the factory reset is doing the leds should flash all red.

Different usb stick was a solution for me.